Add the sample code below to your child theme's functions.php file or in a site specific plugin.
<?php
//* Do NOT include the opening php tag
//* Add a label for the promotion code field
add_filter( 'FHEE__EED_Promotions___add_promotions_form_inputs__ee_promotion_code_input__html_label_text', 'ee_registration_checkout_promotions_heading' );
function ee_registration_checkout_promotions_heading() {
return 'Enter your Discount Code';
}
References:
https://eventespresso.com/topic/ee4-promotional-code/
https://eventespresso.com/topic/ee4-checkout-page-promo-code/
https://eventespresso.com/topic/promo-code-label-missing/
https://eventespresso.com/topic/add-custom-text-or-disclaimer-to-registration-checkout-page-ee4/
https://eventespresso.com/topic/ee4-promotion-header-missing-during-checkout/
https://eventespresso.com/topic/promotion-code-area-just-a-search-bar/
https://eventespresso.com/topic/adding-text-below-promotion-input/#post-232086