Don't know the first, but certainly well used in WWI.
Megan, do you have a reference for that? I cannot find any WW1 aircraft actually fitted with trim tabs as we know them, although I understand that some had various forms of control surfaces stabilisers.
On the SE5 and SE5A the entire tailplane pivoted on the front spar, and the alteration in the AoA of the tailplane was changed by the winding of the "trim wheel" on the LHS of the cockpit - which trim wheel operated a chain, that spun a threaded cog on an acme threaded shaft, contained within the hollow sternpost.
The movement of the cog up or down the threaded shaft changed the relative position of the tailplane rear spar.
I can only find references to trim tabs appearing in aircraft designs from the early 1920's.