PureEngage On-Premises

Is it possible to create a button on " InteractionVoiceCustomButtonRegion " to make a transfer directly to a routing point ?

Now I have the button created , but I can not find a way to make the transfer

At a high level, you'll need to get your button to execute a chain of command. See the reference here: http://docs.genesys.com/Documentation/IW/latest/Developer/Voice

You'll need to invoke the command "InteractionVoiceSingleStepTransfer" and make sure you pass the correct parameters. Something like this:
 

var parameters = new Dictionary<string, object>
                {
                    {"CommandParameter", interactionVoice},
                    {"Destination", routePoint},
                    {"Location", ""},
                    {"UserData", null},
                    {"Reasons", null},
                    {"Extensions", null}
                };
ContainerAccessPoint.Container.Resolve<ICommandManager>().GetChainOfCommandByName("InteractionVoiceSingleStepTransfer").Execute(parameters);

You'll need to get the current interaction (use the IInteractionManager type) and have the route point number handy (store it in the annex and use IConfigManager to fetch it).

Hope that helps.