first step:

POST /botframework.com/oauth2/v2.0/token HTTP/1.1
Host: login.microsoftonline.com
Content-Type: application/x-www-form-urlencoded

client_id=<client id>&scope=https%3A%2F%2Fapi.botframework.com%2F.default&client_secret=<client secret>&grant_type=client_credentials

--------------

second step:

GET /emea/v3/conversations/<thread id>/pagedmembers?pageSize=10 HTTP/1.1
Host: smba.trafficmanager.net
Authorization: Bearer <tokent from step1>