It's not entirely clear what you want, but.. add the keyboard plugin and do
Keyboard: On F pressed --> <do action>
to detect the 'F' key (I'm sure you already know that) but as far as 'only when standing over walkway', can you check your character's Y position? Or have an invisible sprite slightly larger that your character and check for overlapping with the walkway? Is that what you mean? Then use the 2 conditions together.