Token endpoint for chat message

I need to get a token to be able to send messages to a one-one-one chat to another user. I used a Graph Explorer provided token and it worked just fine.

When I try to get a token using Postman @ https://login.microsoftonline.com/{tenant_ID}/oauth2/v2.0/token endpoint, I got an error reply with

. I'm using the scope according to https://docs.microsoft.com/en-us/graph/auth-v2-service?view=graph-rest-1.0#4-get-an-access-token doc instructions and using the scope I copied from my app description page. Could someone send a example of token parameter usage?

If I use the default scope (https://graph.microsoft.com/.default) I got a token, but the message send endpoint returns "UnknownError".

I'm not sure I have a token getting problem, a send message problem or both.

Would some please help? I 've been working on this issue for the last 4 days.