Xero integration using OAuth 2.0

Hi all.

I need to upgrade my Xero integration to use OAuth 2.0 and I'm struggling with 1 element.

I have my Client Id and Client Secret and I can get an Authorization Code but I can't then get the required token.

The Xero documentation says the token request requires:

POST https://identity.xero.com/connect/token
authorization: "Basic " + base64encode(client_id + ":" + client_secret)
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code
&code=xxxxxx
&redirect_uri=https://myapp.com/redirect

My request always returns {"error":"invalid_client"}

I'm using an insert from URL with the following cURL options:

"-X POST" &

" -H \"Content-Type: application/x-www-form-urlencoded\"" &

" -H \"Authorization: \"Basic \""& Base64Encode (ACT_SET::ClientId &":"& ACT_SET::ClientSecret) & "\"" &

" --data-urlencode grant_type=authorization_code" & ¶ & 
" --data-urlencode code="&ACT::AuthorizationCode&¶ & 
"--data-urlencode redirect_uri=https://xxxxxxx.com/"

I'm assuming the issue lies in the syntax of " -H \"Authorization: \"Basic \""& Base64Encode (ACT_SET::ClientId &":"& ACT_SET::ClientSecret) & "\"" & but I've tried everything I can think of and I'm getting no further.

Any help would be much appreciated.