Apply the coupon on the Checkout page via URL.

functions.php

/* 
* How to add the custome code. 
* 
* Please follow below instructions:
* 1. Copy the below code.
* 2. Open your child theme's functions.php file.
* 3. Paste the copied code at the very bottom of it & save the file OR upload it on your server/hosting.
*/

add_action( 'wp', 'wc_add_the_coupon_from_url' );

function wc_add_the_coupon_from_url() {
		
	$discount_coupon = '';

	if( isset( $_GET['token']  ) ){

		$discount_coupon = sanitize_text_field( wp_unslash( $_GET['token'] ) );
	}
	
	if ( is_array( $discount_coupon ) && ! empty( $discount_coupon ) ) {
		$discount_coupon = reset( $discount_coupon );
	}

	if ( ! empty( $discount_coupon ) ) {

		$show_coupon_msg = apply_filters( 'mycustom_filter_action', true );

		if ( ! $show_coupon_msg ) {
			add_filter( 'woocommerce_coupon_message', '__return_empty_string' );
		}

		WC()->cart->add_discount( $discount_coupon );

		if ( ! $show_coupon_msg ) {
			remove_filter( 'woocommerce_coupon_message', '__return_empty_string' );
		}
	}
}