get https://{instance}-api.private.fin.ag/v3/oauth/authorize?institution_id=&redirect_uri=&state=&lang=&tag=
Use the /OAuth/Authorize endpoint if you are using a custom API integration and want to establish a secure connection using OAuth 2.0 technology.
This endpoint responds with a redirect. For more information about how to use this endpoint, see Set up a Custom Integration using OAuth 2.0.
Once the API returns a 302, you can grab the URL from the Header -> Location field and present the end user with the financial institution's login page.