gistfile1.php

<?php

/**
 * [affiliate_referral_url_username] shortcode
 */
function affwp_custom_referral_url_shortcode( $atts, $content = null ) {

	if ( ! affwp_is_affiliate() ) {
		return;
	}

	shortcode_atts( array(
		'url' => ''
	), $atts, 'affiliate_referral_url_username' );

	if ( ! empty( $content ) ) {
		$base = $content;
	} else {
		$base = ! empty( $atts[ 'url' ] ) ? $atts[ 'url' ] : home_url( '/' );

	}

	$affiliate           = affwp_get_affiliate( affwp_get_affiliate_id() );
	$user_info           = get_userdata( $affiliate->user_id );
	$affiliate_user_name = $user_info->user_login;

	return add_query_arg( affiliate_wp()->tracking->get_referral_var(), $affiliate_user_name, $base );
}
add_shortcode( 'affiliate_referral_url_username', 'affwp_custom_referral_url_shortcode' );