create addon

sample.php

<?php
add_action( 'plugins_loaded', 'om_sample_plugins_loaded' );

function om_sample_plugins_loaded() {

	// Bail if the main class does not exist.
	if ( ! class_exists( 'Optin_Monster' ) ) {
		return;
	}

	// Fire up the addon.
	add_action( 'optin_monster_init', 'om_sample_plugin_init' );

}

function om_sample_plugin_init() {

	/**
	 * The 'optin_monster_themes' filter allows for the addition of
	 * new lightbox themes.
	 */
	add_filter( 'optin_monster_themes', 'om_sample_add_lightbox_theme', 10, 2 );

	/**
	 * The 'optin_monster_theme_api' filter directs the new optin request
	 * to the correct theme file.
	 */
	add_filter( 'optin_monster_theme_api', 'om_sample_lightbox_theme_api', 10, 4 );

}