Help to connect to OAuth 2.0 API

Queries and Resources
1

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": "apps.fortnox.se/oauth-v1/token",
    "method": "POST",
    "headers": {
      "Content-Type": "application/x-www-form-urlencoded",
      "Host": "apps.fortnox.se",
      "User-Agent": "Node-oauth",
      "Content-Length": 151
    },
    "body": {
      "grant_type": "authorization_code",
      "redirect_uri": "https://oauth.retool.com/oauth/oauthcallback",
      "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)

2

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:
https://oauth.retool.com/oauth/user/oauthcallback?code=XXXXXXX-XXXX-XXXX-8be4-ca9c2af1301b&state=XXXXXXX-1e69-4ada-87fe-XXXXXXXX

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..)