If you want to add country and region inside the drop-down in custom module admin form, you landed to right page.

Follow below instruction step-by-step to execute it.

Step 1 : Add column in table field.

Create new file : Setup/Installschema.php

<?php

->addColumn(
'country',
MagentoFrameworkDBDdlTable::TYPE_TEXT,
'2M',
['nullable' => false],
'country' 
)->addColumn(
'store_ids',
MagentoFrameworkDBDdlTable::TYPE_TEXT,
'255',
['nullable' => false,
'default' => $defaultstoreid,
],
'store_ids' 
)->addColumn(
'statename',
MagentoFrameworkDBDdlTable::TYPE_TEXT,
'2M',
['nullable' => false],
'statename'
)

?>

Step 2 : Add country and state drop-down with text

Create new file : Block/Adminhtml/Grid/Edit/Form.php

<?php
$optionsc=$this->_countryFactory->toOptionArray();
$country = $fieldset->addField(
'country',
'select',
[
'name' => 'country',
'label' => __('Country'),
'title' => __('country'),
'values' => $optionsc,
]
);

$statename = $fieldset->addField(
'state',
'select',
[
'name' => 'state',
'label' => __('State'),
'id' => 'state',
'title' => __('state'),
'class' => 'required-entry',
'required' => false,
'values' => ['--Please Select State--'],
]
);
$fieldset->addField(
'statename',
'text',
[
'name' => 'statename',
'label' => __('State'),
'id' => 'statename',
'title' => __('state'),
'class' => 'statename',
'required' => false,
]
);

/*
* Add Ajax to the Country select box html output
*/
$country->setAfterElementHtml(" 
<script type="text/javascript">
require([
'jquery',
'mage/template',
'jquery/ui',
'mage/translate'
],
function($, mageTemplate) {
jQuery('.field-statename').hide();

jQuery('#addressbook_country').change(function(){
var conceptName = jQuery('#addressbook_country').find(':selected').val();
jQuery.ajax({
url : '". $this->getUrl('grid/lists/regionlist') . "?country=' + conceptName,
data: conceptName,
type: 'GET',
dataType: 'json',
showLoader:true,
success: function(data){
// console.log(data.htmlconent);
if(data.htmlconent==''){
jQuery('.field-statename').show();
jQuery('.field-state').hide();
}else{
jQuery('#addressbook_state').empty().append(data.htmlconent);
jQuery('.field-state').show();
jQuery('.field-statename').hide();

}

}
}); 
});

}

);
</script>"
);

$statename->setAfterElementHtml(" 
<script type="text/javascript">
require([
'jquery',
'mage/template',
'jquery/ui',
'mage/translate'
],
function($, mageTemplate) {
jQuery('.field-statename').hide();
jQuery(window).load(function(){
setTimeout(function(){
var statename ='".$model->getState()."';
jQuery('#addressbook_state option').each(function (a, b) {
if (jQuery(this).text().toLowerCase() == statename.toLowerCase() ){
jQuery(this).attr('selected','selected');
jQuery(this).trigger('change');

}
});
}, 1000);
});



var conceptName = jQuery('#addressbook_country').find(':selected').val();
if(conceptName != ''){
jQuery('#addressbook_country').trigger('change');
}
}

);
</script>"
);

?>

Step 3 : Getting region list from controller.

Create new file : Controller/Adminhtml/Lists/Regionlist.php

/**
* @SuppressWarnings(PHPMD.CouplingBetweenObjects)
*/
class Regionlist extends MagentoFrameworkAppActionAction
{
/**
* @var MagentoFrameworkViewResultPageFactory
*/
protected $resultPageFactory;
/**
* @var MagentoDirectoryModelCountryFactory
*/
protected $_countryFactory;

/**
* @param MagentoFrameworkAppActionContext $context
* @param MagentoFrameworkViewResultPageFactory resultPageFactory
*/
public function __construct(
MagentoFrameworkAppActionContext $context,
MagentoDirectoryModelCountryFactory $countryFactory,
MagentoFrameworkViewResultPageFactory $resultPageFactory
)
{
$this->_countryFactory = $countryFactory;
$this->resultPageFactory = $resultPageFactory;
parent::__construct($context);
}
/**
* Default customer account page
*
* @return void
*/
public function execute()
{
$countrycode = $this->getRequest()->getParam('country');

if ($countrycode != '') {
$statearray =$this->_countryFactory->create()->setId($countrycode)->getLoadedRegionCollection()->toOptionArray();
$state = '';
if(count($statearray) > 0){
// $state .= "<option value=''>--Please Select State--</option>";
foreach ($statearray as $_state) {
if($_state['value']){
$state .= "<option value=".$_state['label'].">" . $_state['label'] . "</option>";
}
}
}
}
$result['htmlconent']=$state;
$this->getResponse()->representJson(
$this->_objectManager->get('MagentoFrameworkJsonHelperData')->jsonEncode($result)
);
} 

}

?>

Step 4 : Save your data.

Create new file : Controller/Adminhtml/Grid/Save.php

<?php

class Save extends MagentoBackendAppAction
{
/**
* @var NamespaceModulenameModelGridFactory
*/
var $gridFactory;

/**
* @param MagentoBackendAppActionContext $context
* @param NamespaceModulenameModelGridFactory $gridFactory
*/
public function __construct(
MagentoBackendAppActionContext $context,
NamespaceModulenameModelGridFactory $gridFactory,
MagentoDirectoryModelCountryFactory $countryFactory
) {
parent::__construct($context);
$this->gridFactory = $gridFactory;
$this->_countryFactory = $countryFactory;
}

/**
* @SuppressWarnings(PHPMD.CyclomaticComplexity)
* @SuppressWarnings(PHPMD.NPathComplexity)
*/
public function execute()
{
$data = $this->getRequest()->getPostValue();
if (!$data) {
$this->_redirect('grid/grid/addrow');
return;
}
try {

$countrycode = $data['country'];
$statearray =$this->_countryFactory->create()->setId($countrycode)->getLoadedRegionCollection()->toOptionArray();
$rowData = $this->gridFactory->create(); 
if($data['state'] == '0' || count($statearray) == 0){

$data['state'] = $data['statename'];
}
$data['store_ids'] = implode(',', $data['store_ids']); 
$rowData->setData($data);
if (isset($data['id'])) {
$rowData->setEntityId($data['id']);
}
$rowData->save();
$this->messageManager->addSuccess(__('AddressBook has been successfully saved.'));
} catch (Exception $e) {
$this->messageManager->addError(__($e->getMessage()));
}
$this->_redirect('grid/grid/index');
}

/**
* @return bool
*/
protected function _isAllowed()
{
return $this->_authorization->isAllowed('Namespace_Modulename::save');
}
}
?>

Well, that’s it. Try to adopt this solution and let us know how it worked for you.