If you want to add a new button on the sales order view page in the admin side then you have to do that by the plugin.

Please follow the below steps.

Step 1: Create a di.xml file on the below path and paste the below code in it.
Path: app/code/Magemonkeys/CustomOrderButton/etc/adminhtml/di.xml

<?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="MagentoBackendBlockWidgetButtonToolbar">
        <plugin name="add_custom_button_toolbar" type="MagemonkeysCustomOrderButtonPluginCustomButtonPlugin" />
    </type>
</config>

Step 2: Create the plugin file on the below path and paste the below code in it.
Path: app/code/Magemonkeys/CustomOrderButton/Plugin/CustomButtonPlugin.php

<?php declare(strict_types=1);

namespace MagemonkeysCustomOrderButtonPlugin;

use MagentoSalesBlockAdminhtmlOrderCreate;
use MagentoFrameworkViewElementAbstractBlock;
use MagentoBackendBlockWidgetButtonButtonList;
use MagentoBackendBlockWidgetButtonToolbar as ToolbarContext;

class CustomButtonPlugin
{
    public function beforePushButtons(ToolbarContext $toolbar,AbstractBlock $context,ButtonList $buttonList): array {
        $orderObject = false;
        $layoutName = $context->getNameInLayout();
        if ('sales_order_edit' == $layoutName) {
            $orderObject = $context->getOrder();
        }

        if ($orderObject) {
        	$url = 'My Custom URL'
	        $buttonList->add('custom_btn',['label' => __('Custom Button'),'on_click' => sprintf("location.href = '%s';", $url),'class' => 'primary custom_button','id' => 'custom_btn']);
        }

        return [$context, $buttonList];        
    }
}
?>