How to assign keys for XP10

Hello

I am new to X-Plane and having difficulty with some things.

I cannot work out how to assign keys to particular functions such as "FMS/key_load". I read a article which describes how to use flight plans and in it the author advises to "Define a key referring to FMS/key_load".

I have only found "FMS/key_load" in Settings/Joystick and Equipment and the "advanced buttons tab". It is not listed if I select the "Keys" tab.

Would someone advise how to assign a key to Define a key referring to "FMS/key_load"

Thanks