Trsutpilot generate invitations API: error 403

Question

I am trying to use Trustpilot's "Generate invitation links using Trustpilot APIs" for service review links. I managed to get the BUID and access_token, however when trying to generate the link I get an empty 403 error. I have tried several different setups including leaving out the API key and using Basic instead of Bearer but that then just leads to 401 errors. I also checked that the access_token hasn't expired.

#get the unique link
access_token = json.loads(response.text)['access_token']
businessUnitId = "XXX"

invitation_url = "https://invitations-api.trustpilot.com/v1/private/business-units/{businessUnitId}/invitation-links"
payload = {
  "referenceId": "inv00001",
  "email": "[email protected]",
  "name": "John Doe",
  "locale": "en-US"
}

head={"apikey": "secret_api_key",
      "Authorization": "Bearer {access_token}"
      }

response = requests.post(url = invitation_url, data = json.dumps(payload), headers = head)
status_code = response.status_code

I am relatively new to APIs so would really appreciate any kind of hint what is going wrong here! Thanks!