If you display a script in the Scripts menu you will see that it assigns Ctrl-1, Ctrl-2 to each displayed script. You can change the order of their display to select which number your script is assigned.
That's one simple way.
For a given layout, you can use the OnKeystroke script trigger to run a script each time a key is pressed. This script can check to see if the "hot key" (possibly combined with a modifier key such as shift or control) was pressed and respond appropriately. Useful items to use in this script:
Get ( Triggerkeystroke )
Get ( TriggerModifierKeys )
Code () //use to identify keys for invisibile characters such as tab or return
Exit Script [false] //cancels out data entry of key stroke when it was a "hot key".
With FileMaker advanced, you can also define a custom menu and assign short cut key combinations to perform the scripts listed in the custom menu. (Must be shortcuts not already used in system.)