Usage:
add_action( 'arm_after_apply_coupon_code', 'your_function', 10, 2);
function your_function($coupon_code, $plan_id) {
    //Do Your Action
    //$coupon_code Applied Coupon Code
    //$plan_id Plan ID.
}
Parameters:
$coupon_code
Applied Coupon Code
$plan_id
Plan ID
Usage:
add_action( 'arm_before_apply_coupon_code', 'your_function', 10, 2);
function your_function($coupon_code, $plan_id) {
    //Do Your Action
    //$coupon_code Applied Coupon Code
    //$plan_id Plan ID.
}
Parameters:
$coupon_code
Applied Coupon Code
$plan_id
Plan ID