To add a custom button on the admin order view page, you need to create a before plugin for the PushButtons() method from the MagentoBackendBlockWidgetButtonToolbar class.

Create di.xml file under the adminhtml scope,
Path: app/code/Magemonkeys/OrderViewButton/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">
    <!-- Add new Button on sales order view toolbar -->
    <type name="MagentoBackendBlockWidgetButtonToolbar">
        <plugin name="add_sales_button_toolbar" type="MagemonkeysOrderViewButtonPluginBlockWidgetButtonToolbarPlugin" />
    </type>
</config>

create ToolbarPlugin.php
Path: app/code/Magemonkeys/OrderViewButton/Plugin/Block/Widget/Button/ToolbarPlugin.php

<?php declare(strict_types=1);

namespace MagemonkeysOrderViewButtonPluginBlockWidgetButton;

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

class ToolbarPlugin
{
    /**
     * @param ToolbarContext $toolbar
     * @param AbstractBlock $context
     * @param ButtonList $buttonList
     * @return array
     */
    public function beforePushButtons(
        ToolbarContext $toolbar,
        AbstractBlock $context,
        ButtonList $buttonList
    ): array {
        $order = false;
        $nameInLayout = $context->getNameInLayout();
        if ('sales_order_edit' == $nameInLayout) {
            $order = $context->getOrder();
        }

        if ($order) {
        	$url = "YOUR_URL"; // add your full url
	        $buttonList->add(
	            'my_button',
	            [
	                'label' => __('My Button'),
	            	'on_click' => sprintf("location.href = '%s';", $url),
	                'class' => 'primary',
	                'id' => 'my_button'
	            ]
	        );
        }

        return [$context, $buttonList];        
    }
}<code>

Output :