[WordPress] A simple example for how to include a template part from a plugin using `the_content` hook.

acme-add-template-part.php

<?php

add_action( 'the_content', 'acme_add_template_part' );
function acme_add_template_part( $content ) {

	ob_start();
	
	require plugin_dir_path( dirname( __FILE__ ) ) . '/includes/templates/acme-my-template.php';
	$template = ob_get_contents();
	$content .= $template;
	
	ob_end_clean();

	return $content;

}

acme-my-template.php

<?php
/**
 * Template Name: Acme Template Part
 *
 * @since     1.0.0
 */
?>

<form action="#" method="post">

	<p>
		<label for="acme-firstname">First Name <span class="acme-required">*</span></label>
		<input type="text" id="acme-firstname" name="acme-firstname">
	</p>

	<p>
		<input type="submit" value="Submit">
	</p>
	
</form>