If you want to add a custom button in the admin order view, then you can follow below steps.

1. Create file di.xml on app/code/Magemonkeys/Custombutton/etc

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="MagentoBackendBlockWidgetContext">
        <plugin name="add_custom_button_sales_veiw" type="MagemonkeysCustombuttonPluginWidgetContext" sortOrder="1"/>
    </type>
</config>

2. Create file Context.php on app/code/Magemonkeys/Custombutton/Plugin/Widget

<?php
namespace MagemonkeysCustombuttonPluginWidget;

class Context
{
    protected $urlBuider;
    public function __construct(
        MagentoFrameworkAppActionContext $context,
        MagentoFrameworkAppRequestHttp $request,
        MagentoFrameworkUrlInterface $urlBuilder
    ) 
    { 
        $this->urlBuilder = $urlBuilder;
        $this->request = $request;
    }
    public function afterGetButtonList(
        MagentoBackendBlockWidgetContext $subject,
        $buttonList
    )
    {        
        if($this->request->getFullActionName() == 'sales_order_view'){
            $buttonList->add(
                'custom_button',
                [
                    'label' => __('Custom Button'),
                    'onclick' => 'setLocation('' . $this->getCustomUrl() . '')',
                    'class' => 'ship'
                ]
            );
        }

        return $buttonList;
    }

    public function getCustomUrl()
    {        
        $order_id = $this->request->getParam('order_id');
        return $this->urlBuilder->getUrl('modulename/controllername/methodname',array('parameter'=>$order_id));
    }
    
} ?>