If I’ve already filed my taxes but I forgot to add that I recently started a new job, Should I amend them once they are accepted or rejected?

Yes, you should.

You have to wait until the original return is processed and you get your refund.

Then approx mid-February TurboTax will have the Amended process for 2018 returns ready.

You will then log back into your TurboTax account and Amend the 2018 return, adding whatever was left off the original return.