Adding A Tag To Incoming Leads

I am gathering optin leads through a number of sources. I’m collecting the leads and delivering them to clients using a webhook.

One client using Keap, wants the leads “tagged” so he can identify me as the source.

So far, I am to tag the leads as they go into the webhook, but the tag information isn’t arriving in the contact information within Keap. I’m using a Oauth2 connection, because I don’t have login access for the client’s Keap account.

is there a way to apply and send the tag with the contact info?