HotDocs Advance REST API to Service Instance - anyone done this?

Hi,

I'm trying to create a service instance of type REST for incorporating the HotDocs Advance API but not having much joy.

I can query the API fine from PostMan but K2 doesn't like the Auth/Token endpoint.

In HotDocs Advance the call to get Authorized is as follows:

https://hddev.mycompany.com/hdaauth/Authorize/ServicePrincipalLogIn

There's a Header : Cntent-Type = application/json
And the payload is RAW JSON in the Body of the request:
{
"ClientName": "K2Client",
"PrincipalName": "K2ClientSvcPrinciple",
"PrincipalPassword": "passwordhere"
}

As I said this works fine in Postman and I can use the returned token in the BODY to authorize all my calls to the API.

HotDocs have said that their API is not "pure OAUTH"... not sure what that means. Does the above look like it will be able to fit into the K2 REST instance type?

Hope someone can assist?

Regards,

Mike.