From FreeCAD Documentation
- Make sure there is no selection.
- There are several ways to invoke the tool:
- Press the Constrain point on object button.
- Select the Sketch → Sketcher constraints → Constrain point on object option from the menu.
- Use the keyboard shortcut: O.
- The cursor changes to a cross with the tool icon.
- Select a single point and a single edge (in any order).
- A constraint is added.
- Optionally keep creating constraints.
- To finish, right-click or press Esc, or start another geometry or constraint creation tool.