The type of constraint is called Displacement. Select a Vertex, and apply a Displacement constraint. You can assign a zero value to one, two or three axis directions. When I say set Z = 0, that means to leave the other two directions Free.
We could do a Google Meet or Skype call where we can talk and screen share ANSYS so you can more quickly learn what you need to know to be successful with this model. If that is of interest, please leave an email address and I will contact you to set up a time.