MemberPress: Require Specific Coupon to Register

function mepr_only_with_coupon( $errors ) {

  $membership_id = 123; //Change this ID to your membership ID

  $coupons = array( 'IMRECOMMENDED', 'DISCOUNTED' ); //Change these titles to your coupon title(s)

  if( $_POST['mepr_product_id'] == $membership_id ) {

    if( !isset( $_POST['mepr_coupon_code'] ) || empty( $_POST['mepr_coupon_code'] ) || !in_array($_POST['mepr_coupon_code'], $coupons, false ) ) {

      $errors[] = "You must enter a valid coupon code to purchase this Membership";

    }

  }

  return $errors;

}

add_filter( 'mepr-validate-signup', 'mepr_only_with_coupon' );