Applying a MailGroup / MailCode via REST?

Hi,

I've been trying to figure out how to add a MailGroup/MailCode to an address via REST but no where in the documentation can I find how to apply this?

There is mention of the MailGroup here:
https://developer.imis.com/reference/country

Which suggests to me this should be being applied to the Address based on the country code? However when a 'Party' is created via REST the MailGroup does not appear to be being added?

When I do GET requests to Party again there is no mention of MailGroup / MailCode?

Can someone please suggest where I should be looking to apply a MailGroup / Mail Code to a Party Address?

Thanks