If it is a dispatch system, you can only sign a contract with a labor dispatch company. The laborer has a labor relationship with the dispatching company and has no contractual relationship with the work unit.

Labor Contract Law Article 58 The labor dispatch unit is the employing unit referred to in this Law and shall fulfill the obligations of the employing unit to the laborers. The labor contract concluded between the labor dispatch unit and the dispatched worker shall not only specify the matters specified in Article 17 of this Law, but also specify the employing unit, dispatch period and post of the dispatched worker.