Create a custom shortcode that calls and display content from another file/tempalate part

functions.php

<?php
/*
* Create a custom shortcode that calls and display content from another file/tempalate part
*
* We have a file called custom-form.php inside inc folder which contains our custom form codes
*
*/
function custom_form( $atts, $content, $shortcode_tag ){
  ob_start();
  get_template_part('inc/custom-form');
  $form = ob_get_contents();
  ob_end_clean();
  return $form;
}
add_shortcode( 'custom-form', 'custom_form' );
/*
* Now we can just use [custom-form] shortcode to call the custom form
*/
?>