Stuck with tangent function in tutorial

Feeling silly but halfway through the tutorial, where I’m asked to create a rectangle on the surface, I’m unable to select the edge of the rectangle and sketch as directed and choose the Tangent constraint. It just wants to create a new rectangle. If I do succeed in selecting both edges, the constraint

menu goes away! The visual in the video for selection is a little different from what I’m seeing (green dot in video, purple dot for me), so it feels like I may be in a different mode, but I’ve followed the video meticulously and can’t get past this step. Help!

To clarify, I’m referring to 6:15 in this video:

It should be work, but at first double tap on the face, then draw the rectangle.
But its an another question where is the centerpoint of the face…

Purple dots on ipad, the green may be on mac.