Help to connect to OAuth 2.0 API

Trying to connect to API that must be authenticated with 0auth2.
Very good documentation from Sverigeledande på webbaserad bokföring och fakturering

Have tried the standard OAUTH 2 authentication but does not work.

I get to log in to Fortnox and accept the connection to app but then I get following response

untered an error (500):
  "error": "{\"statusCode\":400,\"data\":\"{\\\"error\\\":\\\"invalid_grant\\\",\\\"error_description\\\":\\\"Authorization code doesn't exist or is invalid for the client\\\"}\"}",
  "request": {
    "url": "",
    "method": "POST",
    "headers": {
      "Content-Type": "application/x-www-form-urlencoded",
      "Host": "",
      "User-Agent": "Node-oauth",
      "Content-Length": 151
    "body": {
      "grant_type": "authorization_code",
      "redirect_uri": "",
      "client_id": "G8RZoNl1652x",
      "client_secret": "---sanitized---",
      "code": "---sanitized---"
Please try again

I guess it is because I must have State, Access_type and Response_type parameters in the authorisation request. Is that any way to add this without making a custom Api integration (which seems very complicated)

Ok it seems like the authorisation code is sent as a parameter from the service (Fortnox) to the redirect url of retool. But it can not handle it because I get the error above. In the address field I can see the token though:

So I could basically hard copy the code and proceed to asking for a token with it, but its not a beatuful solution..

Any suggestions of how to proceed? (as you probably understand I am not a developer..)