ninja-forms-webhooks-auth.php

<?php
/*
Plugin Name: Ninja Forms - Webhooks Auth Headers
*/

add_filter( 'nf_remote_post_args', 'nf_wh_add_auth' );

function nf_wh_add_auth( $args ){

    $headers = array(
        'headers' => array(
            'Authorization' => 'Basic ' . base64_encode( YOUR_USERNAME . ':' . YOUR_PASSWORD )
        )
    );

    $args = array_merge( $args, $headers );

    return $args;

}