Answer:
Form-finding with prestress can be activated in General Data of the surface by clicking the "Edit Parameters of Stiffness" button.
When activating the RF‑FORM‑FINDING add-on module (in General Data of the model), the "Form-Finding" tab is visible for surfaces with one of the following stiffness types:
- Standard
- Orthotropic
- Membrane
- Membrane – Orthotropic
Generally, the program automatically activates the form-finding with the standard prestress for surfaces with the stiffness types "Membrane" and "Membrane – Orthotropic".