Yes.. There would be two ways that you can obtain user name
APIM would return the user name with the Access token. Here user name would be embedded with the access token. You can find more details from here
You can obtain the user name and user's attribute during access token verification process. Once access token is verified successfully, It would return back with JWT token which contains user's attributes. Then you can extract use name from JWT. You find more details on here. Basically inside the APIM, you can extract JWT token in transport header. But, if you want to retrieve the JWT token for the use of your application, you can send the access token to "OAuth2TokenValidationService" service and validate it and then it return back with JWT. (But "OAuth2TokenValidationService" is a web service.). You can use web service client to invoke this service (such as SOAPUI)