Register Merge Tags example

addon_base_class.php

<?php

public function __construct()
{
    /*
     * Required for all Extensions.
     */
    add_action( 'admin_init', array( $this, 'setup_license') );

    // Call our register merge tags method. 
    add_action( 'ninja_forms_loaded', array( $this, 'register_mergetags' ) );
}

/**
 * Register Merge Tags Call Back
 */
public function register_mergetags()
{
    Ninja_Forms()->merge_tags[ 'merge_tag_example' ] = new NF_MergeTagExample_MergeTags();
}