This is a video for an F250 but it appears there is a spot under a cup holder (or for other cars in a glovebox maybe?) to place the key.

Do you guys know where this is for a Mustang though? How about getting into the car with a dead battery fob?

 

Vehicles equipped with Sync Connect, 2016 model year forward, can remotely unlock and start the vehicle
using the Sync Connect app. Owners need to enable SYNC Connect through a two-step authentication
process to protect personal information, confirming setup on the vehicle touch screen and on the mobile
application.
https://owner.ford.com/support/how-t...-fordpass.html

Ford, it can be anywhere, console, under cup holders, or glove box, a place to put the keyfob to be powered by
the induction coil. Ford has not standardized it...

Foreign cars, you place the fob on or very close the start button, and it will work.

FORD
You pop the back of your FOB off, push the tab in and pull it off, and there should be a key inside it.
Then, at the door, there is a slot on the underside of the door handle, insert the key and press upward,
to remove the cover by sliding it right to expose the key hole. There should then be a slot under the
armrest in the center console, left side. Place the fob in the slot, with the buttons facing forward.

 

Mines under the cup holder. Just pull the mat out. Entry into the vehicle is exactly as described above.

 

Yeah, new Stangs are under the cupholder mat;
My car is the front cupholder,
my dads has a place in the center console where it can sit in...
each vehicle is different

 

For stangs they are under the cup holder mat . I keep my spare key with me in a ford fob key cover with me .