If the In-plan Roth Rollover occurred in 2018, your 401(k) plan is required to have issued to you a 2018 Form 1099-R reporting this. It must have code G in box 7.
If you don't have the Form 1099-R from the plan, contact them to obtain the missing form. Don't try to complete the 1099-R entry without it. (If you haven't received the Form 1099-R, it might be because the transaction did not occur until 2019, in which case it's 2019 income, not 2018 income.)
