how do I use a group/group password to update a ddns entry

I need to create multiple subdomains, each with its own password to update (so I do not expose my password in customer's routers)

i.e.:
loc1.mydomain.net - group cust1 custpass1
loc2.mydomain.net - group cust2 custpass2
etc...

no matter what I've tried so far, the result of the update url (i.e: http://api.dynu.com/nic/update?hostname=loc1.mydomain.net&username=cust1&password=custpass1) is "badauth"