How save AdditionalField data for a domain in WHMCS

I am having a hard time figuring out how does the Additional fields for Domains even work other than being editable on the order form and from admin panel, but what if we have to update its custom fields from a Module such as Registrar module.

I have a requirement where my domain registrar sends a Request ID in its domain register API response, which I have to store with the domain to later fetch domain status using it.

I have created an Additional field for domain


$additionaldomainfields[".co"][] = array(
"Name" => "RequestId",
"Type" => "text",
"Show on Order Form" => false
);

And have `AfterRegistrarRegister` hook in my registrar module.


add_hook('AfterRegistrarRegister', 1, function($vars) {

require_once 'init.php';
$varsjson = json_encode( $vars );
$domainName = $vars['params']['sld'] . '.' . $vars['params']['tld'];
$domainid = $vars['domainid'];





//return array(
// 'abortWithSuccess' => true,
//);
}
);

I tried

$vars['additionalfield']['RequestId'] = 'testtt';

But of course it doesn't work.

I am also referring to the Classdocs of WHMCS

[To see links please register here]

But there appears to be no method that updates the values of a domain additional field.

Any help is appreciated.