The movement of funds from a traditional 401(k) account straight to a Roth IRA is defined in the tax code as a taxable rollover, not a Roth conversion. The term "Roth conversion" only applies to a movement of funds from a traditional IRA to a Roth IRA. A 401(k) is not an IRA. Because a rollover of a traditional 401(k) to a Roth IRA is not a Roth conversion, it is not reportable on Form 8606.
Note that the tax code does permit the movement of funds from a traditional 401(k) to a Roth IRA, but some plan reps are still not aware of this despite it being permissible for more than 10 years. They might mistakenly suggest that you must first roll the traditional 401(k) over to a traditional IRA and then do a Roth conversion from the traditional IRA, which would be reportable on Form 8606, but rolling first to a traditional IRA is not necessary.
To avoid mandatory tax withholding you must do the rollover from the 401(k) as a direct rollover where the plan makes the distribution payable directly to the receiving IRA for your benefit. By making the distribution payable to the particular IRA rather than just to the custodian for your benefit it avoids the possibility that the custodian deposits the funds into the wrong type of account. This means that the receiving account needs to already be established and then is subsequently funded by the rollover.
