Power Query REST API error

Change your data source setting accordingly.  You need to clear permissions as they are cached.

The error occurs because Web.Contents with Content requires a RelativePath. Use this fixed script:

let
    base_url = "https://api.qgenda.com",
    url_login = "/v2/login",
    body = [
        email = "emailaddress",
        password = "password"
    ],
    encodedBody = Text.ToBinary(Uri.BuildQueryString(body)),
    response_login = Web.Contents(base_url, [
        RelativePath = url_login,
        Headers = [#"Content-Type" = "application/x-www-form-urlencoded"],
        Content = encodedBody
    ]),
    jsonResponse = Json.Document(response_login),
    accessToken = jsonResponse[access_token],
    OutputToken = accessToken
in
    OutputToken