To add a Dwell control like on the standalone reverb to a Fender amp with reverb, remove the 1 meg resistor and replace it with a pot.

One end of the pot goes to ground and the other end goes to the capacitor - just like it is now.

But the wiper (middle lead) of the pot goes to the grid (pin 2 and 7) of the tube.

The existing wire goes under the board and attaches to the existing 1 meg resistor and 500 pf cap.