You'll need a parallel process in the corner of all of your maps checking if you've pressed the 1 key.
So, make your event, set it to parallel process, and set up a key input process (Page 3). Take a look at this picture, and note that pressing the 1 key sets the variable to 11 (it'll explain why there's a conditional branch set up like that later).
The rest is really simple. Make a conditional branch where the condition is if [whatever variable you saved your key input to] is 11, then open your save menu.