You do not complete the Form 1099-R manually without receiving the Form 1099-R from the plan administrator for the 401(k) tax deferred retirement account.
If the withdrawal was in 2019 then you will not receive the 1099-R until January 2020 for entry on the 2019 tax return in 2020.
If the withdrawal was in 2018 then you should have received the 1099-R in January 2019. If you do not have the 1099-R then contact the plan administrator or sign onto the plan website and get the 1099-R.
