Issues with OIDExternalUserAgent when requesting OIDAuthState

I switched to GTMAppAuth from AppAuth iOS as it hasn't been updated for a long time. I am struggling to get the OIDAuthState: Earlier i used to get it as follows:

OIDAuthState.authState(byPresenting: request, presenting: self) { authState, error in
if let authState = authState, let accessToken = authState.lastTokenResponse?.accessToken

But now with the GTMAppAuth library it is asking for one more parameter external user agent as follows:

OIDAuthState.authState(byPresenting: request, externalUserAgent: self, callback: { state, error in
})

And it is giving me errors with this parameter. I'm not sure why that parameter is in the call as there is no mention of it in the documentation and what to pass in external user agent? Can you please help me with this?