Flat-Footed

[YAKO SOMEDAKY]

If I made a effect

Flat-Footed; GRANTCA; ADVATK; [Target]; [Roll]

The target loses CA Bonus and I make attacks with advantage against the target?

[OneSidedDie]

If you are trying to give attackers advantage on attacks against this target use GRANTADVATK
If you are trying to have the target get disadvantage on its attacks use DISATK

You may be looking at the 4E effects.

https://www.fantasygrounds.com/wiki/index.php/5E_Effects

[Zacchaeus]

I'd suggest that the easiest way to handle this kind of one-off thing is to just use the ADV and DIS buttons on the bottom left of the screen before the rolls are made.

[Skellan]

Yeah, or perhaps remove their Dex bonus to ac. Not sure how you would do that as an effect though.

[Zacchaeus]

Since Flat Footed isn't in the 5e ruleset (at least as far as I can see) I presume that this is some kind of house rule. At any rate since it isn't something that is in the ruleset it's not something that will automate within FG. If it were possible the syntax for reducing the AC by the DEX bonus would be something like AC: [-DEX]. However the AC keyword only supports numbers. You could set up an effect for every player which used AC: -x where x equals their Dex bonus, then as OneSidedDie says above use GRANTADVATK on any enemies targeting that PC or creature. This should automate everything for you.

One word of caution however; because of the Bounded Accuracy inherent in 5e, creating something like this could be over powerful and your PCs might all end up dead in the first encounter. The surprise rules already provide a penalty for characters and monsters who are surprised and that might well be enough without trebly penalising them for failing a surprise roll.