Another example for building your own custom variables

custom_variables_two.php

add_action( 'fue_before_variable_replacements', 'fue_register_variable_replacements', 10, 4);

function fue_register_variable_replacements( $var, $email_data, $fue_email, $queue_item ) { 
    $variables = array(
    	'referral_code'	=> ''
	);

    if ( isset( $email_data['test'] ) && $email_data['test'] ) {
        $variables['referral_code'] = '13221';

        if ( !empty( $email_data['order_id'] ) ) {
            $order = wc_get_order( $email_data['order_id'] );
            $referral_code = get_user_meta( $order->customer_user, '_affiliate_key', true );

            if ( !empty( $referral_code ) ) {
                $variables['referral_code'] = $referral_code;
            }
        }
    } else {
        if ( !empty( $queue_item->order_id ) ) {
            $order = wc_get_order( $queue_item->order_id );
            $referral_code = get_user_meta( $order->customer_user, '_affiliate_key', true );

            if ( !empty( $referral_code ) ) {
                $variables['referral_code'] = $referral_code;
            }
        }
    }

    $var->register( $variables );
}

I'm having the same problem as dacgarz. Is there an update on this issue? I'm not able to add a custom variable now.

not working for me either. When I add the code and test it on the site it gives me the test data in a preview, but is totally blank on email