n8n will handle it for you. You don’t need to make the initial request to get the refresh token.
When activating the credential, it will call the request api and have you authenticate, then when you’re redirected back to the redirect URI (in n8n) it will grab the refresh token and manage it for you.
There are some instances where i have seen this not work, like in some Zoho apps for instance, and in those cases i sometimes handle the oauth logic manually like i show here