BLOCK-LEVEL ON ERROR UNDO, THROW.

USING System.*.
USING System.IO.*.
USING RestSharp.*.
USING RestSharp.Authenticators.*.

DEFINE VARIABLE chSubject AS CHARACTER NO-UNDO.
DEFINE VARIABLE chBody AS CHARACTER NO-UNDO.
DEFINE VARIABLE varemail AS CHARACTER NO-UNDO.
DEFINE VARIABLE vartxser AS CHARACTER NO-UNDO.
DEFINE VARIABLE ix AS INTEGER NO-UNDO.

def var varclie as RestSharp.RestClient.
def var varrequ as RestSharp.RestRequest.
def var varuri as System.Uri.
DEF VAR varerr AS System.Exception.

do on error undo, throw:
assign chSubject = "test"
chBody = "testing"
varemail = "test@xyz.com".

varuri = NEW Uri ("api.eu.mailgun.net/v3"").
varclie = NEW RestClient ().
varerr = NEW Exception ().

varclie:BaseUrl = varuri:AbsoluteUri.
varclie:Authenticator = 
NEW HttpBasicAuthenticator ("api", "ffd50f5cbf796e6dd0827a1ef0674dc4-f45b080f-164170d7").
varrequ = NEW RestRequest (). 
varrequ:AddParameter("domain","xyz.net", ParameterType:UrlSegment).
varrequ:Resource = "~{domain}~/messages".
varrequ:AddParameter("from", "admin@xyz.net").
varrequ:AddParameter("to", varemail).
varrequ:AddParameter("subject",chSubject).
varrequ:AddParameter("text",chBody).
varrequ:Method = Method:POST.

varclie:Execute(varrequ).

CATCH e AS Progress.Lang.Error :
MESSAGE e:tostring()
VIEW-AS ALERT-BOX.
MESSAGE e:GetMessage(1) skip e:CallStack
VIEW-AS ALERT-BOX. 
END CATCH.
end.

using System;
using RestSharp;

public class SendSimpleMessageChunk
{

    public static void Main(string[] args)
    {
        Console.WriteLine(SendSimpleMessage().Content.ToString());
    }

    public static IRestResponse SendSimpleMessage()
    {
        RestClient client = new RestClient();
        //client.BaseUrl = new Uri("https://api.mailgun.net/v3"");
        client.BaseUrl = "https://api.mailgun.net/v3"";
        client.Authenticator = new HttpBasicAuthenticator("api","YOUR_API_KEY");
        RestRequest request = new RestRequest();
        request.AddParameter("domain", "YOUR_DOMAIN_NAME", ParameterType.UrlSegment);
        request.Resource = "{domain}/messages";
        request.AddParameter("from", "Excited User <mailgun@YOUR_DOMAIN_NAME>");
        request.AddParameter("to", "test@xyz.com");
        request.AddParameter("to", "YOU@YOUR_DOMAIN_NAME");
        request.AddParameter("subject", "Hello");
        request.AddParameter("text", "Testing");
        request.Method = Method.POST;
        return client.Execute(request);
    }
}

The ABL code is throwing an ambiguous exception because in the RestSharp.RestClient class there is an Execute method and there is also a generic method Execute<T>. They both take the same number and type of parameters. The AVM executes all method calls and properties getters/setters via reflection. First we call GetMethod() on the System.Type object for the type of the class that owns the method. We tell it what the method name is and the parameter types that we will be passing in. When we do that, .NET throws an AmbiguousMatchException. What that means is that there are 2 methods with the same name whose parameters match and it doesn't know which one you want.

4GL Trace:

4GL 4GLMESSAGE System.Reflection.AmbiguousMatchException: Ambiguous match found. 4GL 4GLMESSAGE at System.DefaultBinder.SelectMethod(BindingFlags bindingAttr, MethodBase[] match, Type[] types, ParameterModifier[] modifiers) 
4GL 4GLMESSAGE at Progress.ClrBridge.UnknownBinder.SelectMethod(BindingFlags bindingAttr, MethodBase[] match, Type[] types, ParameterModifier[] modifiers) 
4GL 4GLMESSAGE at System.RuntimeType.GetMethodImpl(String name, BindingFlags bindingAttr, Binder binder, CallingConventions callConv, Type[] types, ParameterModifier[] modifiers) 
4GL 4GLMESSAGE at System.Type.GetMethod(String name, BindingFlags bindingAttr, Binder binder, Type[] types, ParameterModifier[] modifiers) 
4GL 4GLMESSAGE at Progress.ClrBridge.ClrApi.InvokeMethod(Object instance, String typeName, String methodName, Type[] types, Object[] args) 
4GL 4GLMESSAGE at Progress.ClrBridge.BrgClrFromPro.invokeMethod(BrgClrFromPro* , Int32 A_0, SByte* A_1, SByte* A_2, Int32 A_3, prmhdr* A_4, Int32* A_5, Int32 A_6)