I don't think that there is any valid http request that it is not possible to create with OkHttp.
You can create this request with Job.PostString. The data itself is JSON.
I tried job.PostString using HttpUtils2 library for the following Curl and I got error message as "SSLPeerUnverifiedException:
Nopeercertificate"
............Curl..............
curl -v
https://api.sandbox.paypal.com/v1/oauth2/token \
-H "Accept: application/json" \
-H "Accept-Language: en_US" \
-u "client_id:secret" \
-d "grant_type=client_credentials"
..................................
...........My Code............
Dim Job As HttpJob
Job.Initialize("MyToken",Me)
Job.Username = "AcY........4NJUL"
Job.Password = "EMD.....GplHKi-x"
Job.PostString("
https://api.sandbox.paypal.com/v1/oauth2/token","grant_type=client_credentials")
Job.GetRequest.SetHeader("Accept", "application/json")
Job.GetRequest.SetHeader("Accept-Language", "en_US")
Job.GetRequest.SetContentType("application/json")
Job.GetRequest.SetContentEncoding("text/plain")
................................................