Selecting the top edge of the trapezium is most easily done in Vertex mode - select both ends and switch back to Edge mode. But even then, TinyCad is a bit of a long way round, here.
Vertices must somehow be cut (topologically) into the bottom edge of the rectangle, so alternatives include:
- With Snap set to 'Active' and 'Vertex' and Automerge switched on:
- AltD duplicate the bottom vertices of the rectangle in their edge, and drag them any old where..
- Snap them onto the top vertices of the trapezium
On the whole though, when modeling, you would want the extra edges in the rectangle. If so..
- CtrlR and scroll, cut 2 edge-loops into the rectangle
- With Snap and Automerge set as before, and your Pivot set to 'Median', in Vertex mode, with one of the new loop vertices active...
- SX the new loops, snapping to one of the top vertices of the trapezium.
These are by no means the 'right' ways of doing this.. I'm sure you have/will have your own grab-bag of favorite modeling patterns.