Unless the changed something in Filemaker since the last time I investigated this, you can't. The keys have assigned functions that cannot be overridden.
You can use MBS FileMaker Plugin to trigger scripts by key combinations.
Check the Hotkey functions:
You can just download the plugin and try the examples for Hotkey.
you can't use the function keys, but you can use any other key via Custom Menus (FMP Advanced). Without Custom Menus, you already have ctrl-0 through ctrl-9 available to you (the first 10 checked scripts). With Custom Menus, you can redefine these, or add to them. For example, ctrl-shift-5 might run a script, ctrl-alt-4 to change password, or override ctrl-p to run a special printing script.