From FreeCAD Documentation
- Make sure no dimensional constraints have been selected.
- There are several ways to invoke the tool:
- Press the Toggle driving/reference constraint button.
- Select the Sketch → Sketcher constraints → Toggle driving/reference constraint option from the menu.
- Use the keyboard shortcut: K then X.
- The mode of the dimensional constraint creation tools is toggled:
- In driving mode their menu and toolbar icons are red, and they create driving constraints (default color red). The icon of this tool is then: .
- In reference mode their menu and toolbar icons are blue, and they create reference constraints (default color blue). The icon of this tool is then: .