1) Add an Assign
2) Click Details for the Assign
3) Click new to create a new mapping
4) Map input.payload/input to output.payload/result
5) Click away
6) It will ask you if you want to create an initializer
7) Click Yes
8) It will create a mapping above the "Variable to Variable" called "Fixed Value to Variable"
9) The "Fixed Value to Variable" appears to be selected / highlighted, however what is displayed in the text box is the text for the "Variable to Variable"

It should either update the text box to show the "Fixed Value to Variable" or it should show the "Variable to Variable" selected.