I have some users on a 200EL LW19 who use external forward. they want to activate / deactivate it quickly.

I've never used the Foward External feature key. Can I program a button and then the user dials the speed dial bin? To turn it off just press the button again?

You set up the call fwd always destination by using the Superkey, and then a sped dial bin as the destination.
Program a FWD Always feature key on the phone.
When the light is lit, CFWD Always is enabled, when the light is out, the phone uses 'Global Forwarding' from form 19.

this sounds perfect. thank you!

As far as global forwarding is concerned, I found out this system is an upgraded lw17 to 19 so all call forwarding has to be entered manually. will that affect anything when the cfwd button is not lit?

In that case, your best bet woulbe to give them 2 buttons, on for fwd always, and one for fwd busy/NA.
Whichever one is lit, is the one controlling their phone.

genius!