Yes it is a bit tricky.
Select the top morph face (use the Ctrl+Shift click on the top face), you will know you've got it when the morph has black dots along the top edges, then click on the edge (not a dot) choose the box stretch option ... now when you click on a corner of this highlighted plane your pet palette will have only 2 options. Choose the second one.
In your image, you will see three options in the pet palette. Also you will see that your cursor is aligned with the back face of the morph volume.
PS Another clue is that you don't want to see the morph volume highlighted, only the top plane should be highlighted.