How to set RedirectURI for OAUTH Accesstoken retrival

We have an instance of thinktecture identityserver3 “protecting” some Web.Api Rest and an corresponding WebApplication.
Now we want to test solely the WebApi REST with SoapUI. In that case, what would the redirectURI be? Actually the SoapUiClient would be the redirection target. But the SoapUI Documentation has a strange URI: urn:ietf:wg:oauth:2.0:oob. This does not work, despite this entry is added to the identityservers redirectURI config.

Has anyone tested its identityserver3 protected RestServices with Soap UI? What settings were used for redirectURI?

Thank you