rcp_registration_register_button

Use this filter to change the text on the registration submit button. This will have better results than simply editing the text in the template file directly.

Example:

/**
 * Change the submit button text from "Register" to "Submit Payment".
 * 
 * @param string $button_text Original button text ("Register").
 * 
 * @return string Modified button text ("Submit Payment").
 */
function ag_rcp_registration_register_button( $button_text ) {
    return __( 'Submit  Payment', 'rcp' );
}
add_filter( 'rcp_registration_register_button', 'ag_rcp_registration_register_button' );

Note: If you're using Stripe Checkout then you will need to use a slightly different method for changing the button text:

add_action( 'wp_footer', function() {
	?>
	<script>
		if( typeof rcp_script_options !== 'undefined' ) {
			rcp_script_options.pay_now = 'Custom Text';
		}
	</script>
	<?php
}, 11 );

Change 'Custom Text' to your desired value.