The Create Form tool is available in the mass family template (Mass.rft), the curtain panel pattern based family template (Curtain Panel Pattern Based.rft), and through the In-Place Mass tool in a project. See Creating an In-Place Mass. The access to the Create Form tool varies depending on whether the conceptual design environment is accessed through the family mass template (RFT) or a project file (RVT).
To access Create Form from a project file (RVT)
You can also just double-click in a blank area to finish.
To access Create Form from a mass family template (RFT)
The Create Form tool becomes available on Modify | Form Element tabForm panel. See Creating Solid Forms.