This is an example of a custom functions plugin that sets the 'Submit Promotion Code' button text regardless of the promotions settings.

tw_ee_custom_functions.php

<?php
/*
Plugin Name: Custom EE Functions Plugin
Description: You can add custom functions to this plugin.
Author: Tony Warwick
Version: 1.0
*/
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */

//Filter the Promotion code cubmit button text.
function my_example_promotions_button_text( $text ) {
	    return 'Submit Promotion Code';

}
add_filter( 'FHEE__EED_Promotions___add_promotions_form_inputs__ee_promotion_code_submit__default', 'my_example_promotions_button_text' );

//Filter the promotion code input field label.
function my_example_promotions_html_label( $text ) {
	    return 'Enter your promotion code:';

}
add_filter( 'FHEE__EED_Promotions___add_promotions_form_inputs__ee_promotion_code_input__html_label_text', 'my_example_promotions_html_label' );