How to create templates for a custom module to show on the front end within my theme?

I’d like to create a simple plugin with a form on the front end which submits the value of 3 fields into a DB table.

For the plugin, I’ve been using this answer: https://wordpress.stackexchange.com/a/162476/77891

What I can’t get working now, is a template.

Within the main plugin file, I’ve added:

add_action( 'gm_virtual_pages', function( $  controller ) {      // first page     $  controller->addPage( new \WEBP\Management\Page( '/custom/page' ) )         ->setTitle( 'My First Custom Page' )         ->setTemplate( 'custom-page-form.php' ); } ); 

And my custom-page-form.php:

<?php  echo "Test Echo";  ?> <p>Test</p> <p>Echo</p>  

The title is being displayed (My First Custom Page). But not my Test Echo. How does a correct / working template file have to look like?