Write a detail letter stating that you mistakenly as a matter of oversight, omit or hasn't answer truthfully. Along with an updated sign TRV form. The earlier you do that the better and safer you are.
i did the same mistake with my work visa after realizing that. i sent one webform letter attached with it to visa officer and he considered it and got approved also. dont worry. but raise that webform immediately with one apology letter attached. @U_234
We should also consider expressing CAUTION, while submitting webforms!
IRCC has often said, don't send any additional documents unless asked for; or that they'll not respond unless it's way beyond processing times.