So you decided to use this method. Congratulations, it's the most safest way to to have XMPP messages support, without having to enter you password anywhere.

1. First step you have to do is to go to https://cloud.google.com/console?redirected=true#/project and create a new projects if you do not have it. Just click "CREATE PROJECT" and you are ready.
2. Now just choose your newly created project.
3. After you have opened your project. Navigate to "APIs & auth" > "Credentials"
   
4. Now just click "CREATE NEW CLIENT ID", enter Authorized redirect URI (copy it form xmpp configuration)

Place where redirect uri can be found.

Screenshot where it has to be pasted.

Now just click "Create Client ID"

After that just copy "Client ID" and "Client secret"

The only thing you have to do there is to enter REDIRECT URL whichi you see in

After you have done that, you can just to click "Authentificate and grant permission to send a message". After that you will see a button "Send test message"

So that's all.