Validate custom registratio field in database

Trying to validate a custom form entry against a valid license key stored in a separate database BEFORE a new user is registered. Research has me using a post reply from Jason describing code to be placed in ../wp-content/mu-plugins/s2hacks.php
Upon license failure I’m trying to get an alert(‘Invalid License’) to display before a new user is registered. Step 1 is to get the function to trigger.
Recommendations?

add_action(“init”, “my_custom_validator”, 1);
function my_custom_validator(){
$custom_fields = stripslashes_deep($_POST[s2member_pro_authnet_registration][custom_fields]);
if(empty($custom_fields[“rescue_code”])){ // This custom field is missing?
// Set the error response message for s2Member to display.
$response[“error”] = TRUE;
$response[“response”] = “Custom Field ID

my_unique_field_id

is missing. Please try again.”;
}
}