Connector wont execute on REST call

Hello, i'm currently developing a C# client where I need to instance some process via REST API. I have no problem executing any task with contracts, but when any userTask has a connector (specifically a rest connector to another server of mine), this won't execute and its skipped while the task is completed sending me a success response (Code: 204). However, if I perform the tasks on the portal every connector works perfectly.
The method i'm expecting to execute both the task and connector is {API/bpm/userTask/:taskId/execution} sending the correspondent json with the contract.

How i get a userTaskId?

I searched in Javadocs for a userTaskId method , and i dont find any direct method. its a value from 2 or more method calls? How i reach this value?
My objective is create a Tasks Shelf, anyone who take a task, can return it if dont wanna or simple view that task. and for that i needed a release task, the method needs userTaskId. and when take the task, another tasks became invisible until finish or return a task. using Hide Task method