This plugin drops into the /wp-content/mu-plugins/ folder to add a page that creates a scheduled action for Action Scheduler.

action-scheduler-create-action-page.php

<?php
/**
 * Plugin Name: Add "Create Scheduled Action" page
 * Plugin URI: https://gist.github.com/JPry/357ee26a46e3d8e2b68d1690f5bbbf75
 * Description: Add a page to the Tools menu that allows for the manual creation of scheduled actions with Action
 * Scheduler. Version: 1.0.0
 * Author: Jeremy Pry
 * Author URI: https://jeremypry.com/
 * License: MIT
 */

use ActionScheduler as ActS;

// Prevent direct access to this file
if ( ! defined( 'ABSPATH' ) ) {
	die( "You can't do anything by accessing this file directly." );
}

// Set up a custom page that allows for createing scheduled actions
add_action(
	'admin_menu',
	function() {
		// Ensure we actually have Action Scheduler loaded.
		if ( ! class_exists( ActS::class, false ) ) {
			return;
		}

		$messages = [];

		$hook = add_submenu_page(
			'tools.php',
			'Create Scheduled Action',
			'Create Scheduled Action',
			'manage_options',
			'jpry-create-scheduled-actions',
			function() use ( &$messages ) {
				$hook  = sanitize_text_field( $_REQUEST['hook'] ?? '' );
				$type  = sanitize_text_field( $_REQUEST['type'] ?? '' );
				$group = sanitize_text_field( $_REQUEST['group'] ?? '' );
				$when = sanitize_text_field( $_REQUEST['when'] ?? '' );

				?>
				<h1>Create a Scheduled Action</h1>
				<?php
				foreach ( $messages as $message ) {
					printf(
						'<div class="notice notice-%s"><p>%s</p></div>',
						esc_attr( trim( $message['type'] ) ),
						esc_html( $message['message'] )
					);
				}
				?>

				<div class="wrap">
					<form action="tools.php?page=jpry-create-scheduled-actions" method="post">
						<?php wp_nonce_field( 'jpry-create-scheduled-action' ); ?>
						<table class="form-table">
							<tr>
								<td><label for="jpry-hook">Hook name:</label></td>
								<td>
									<input type="text"
										   name="hook"
										   id="jpry-hook"
										   class="regular-text"
										   value="<?php echo esc_attr( $hook ); ?>"
									>
								</td>
							</tr>
							<tr>
								<td><label for="jpry-group">Group:</label></td>
								<td>
									<input type="text"
										   name="group"
										   id="jpry-group"
										   class="regular-text"
										   value="<?php echo esc_attr( $group ); ?>"
									>
								</td>
							</tr>
							<tr>
								<td><label for="jpry-type">Type:</label></td>
								<td>
									<select name="type" id="jpry-type">
										<option value="async" <?php selected( $type, 'async' ); ?>>Async</option>
										<option value="single" <?php selected( $type, 'single' ); ?>>Single</option>
										<!--<option value="recurring" <?php selected(
											$type,
											'recurring'
										); ?>>Recurring</option>-->
									</select>
								</td>
							</tr>
							<tr>
								<td><label for="jpry-when">When (for single):</label></td>
								<td>
									<input type="datetime-local"
										   name="when"
										   id="jpry-when"
										   value="<?php echo esc_attr( $when ); ?>"
									>
								</td>
							</tr>
						</table>
						<input type="submit" value="Submit" class="button">
					</form>
				</div>
				<?php
			}
		);

		add_action(
			"load-{$hook}",
			function() use ( &$messages ) {
				if ( ! array_key_exists( '_wpnonce', $_REQUEST ) ) {
					$messages[] = [
						'message' => 'No Nonce detected.',
						'type'    => 'info',
					];

					return;
				}

				if ( ! wp_verify_nonce( $_REQUEST['_wpnonce'], 'jpry-create-scheduled-action' ) ) {
					$messages[] = [
						'message' => 'Invalid Nonce!',
						'type'    => 'error',
					];

					return;
				}

				if ( ! isset( $_REQUEST['hook'], $_REQUEST['type'] ) ) {
					$messages[] = [
						'message' => 'Hook name and type are both required.',
						'type'    => 'error',
					];

					return;
				}

				$hook  = sanitize_text_field( $_REQUEST['hook'] );
				$type  = sanitize_text_field( $_REQUEST['type'] );
				$group = sanitize_text_field( $_REQUEST['group'] );
				$when = sanitize_text_field( $_REQUEST['when'] );

				if ( empty( $when ) ) {
					$time = new DateTime( 'now', wp_timezone() );
				} else {
					$time = date_create_from_format( 'Y-m-d*G:i', $when, wp_timezone() );
				}

				switch ( $type ) {
					case 'async':
						ActS::factory()->async( $hook, [], $group );
						$messages[] = [
							'message' => sprintf( 'Added async hook "%s" with group "%"', $hook, $group ),
							'type'    => 'info',
						];
						break;

					case 'single':
						if ( ! $time instanceof DateTime ) {
							$messages[] = [
								'message' => 'Error converting date/time to timestamp.',
								'type'    => 'error',
							];

							return;
						}
						ActS::factory()->single( $hook, [], $time->getTimestamp(), $group );
						$messages[] = [
							'message' => sprintf( 'Added single hook "%s" with group "%"', $hook, $group ),
							'type'    => 'info',
						];
						break;
				}
			}
		);
	}
);