Effect coding for Lesser Magic Resistance

Hi all, I'm starting to play with FGU and I'm having difficulty creating effects.
I would like to create an effect that applies magic resistence only if the PC is proficient on the specific saving throw.
I was wondering if there was a way to create such an effect. Alternatively, it would be enough for me to set magic resistence to a specific enable, IE "magic resistance: constitution,intelligence"

Welcome to FG.

You can't set a limited magic resistance. Magic Resistance will apply advantage to any saving throw from any magic source. There is the Gnome Cunning ability which will automatically give advantage on saving throws against magic which uses Intelligence, Wisdom, or Charisma however that's hard coded and isn't editable. Likewise there's no effect that you can create to test if a PC has proficiency in any particular ability score. So to do what you want you'll either need to write an extension or manually apply advantage to any saving throws that come up with that specific set of conditions.

Thanks a lot for the answer. Extension development seems complicated, maybe I can take a simple one apart to understand how it works.