If you, as the  tax preparer, are naming yourself as the 3rd Party Designee for all returns under Tools, then you are automatically designated that in the return when it's e-filed.

It's confusing because in previous years it was printed on the 1040 that you were the designee, however, now the 1040 states "Other than tax preparer" under the 3RD Party Designee area and also includes, "Do you want to allow another person (other than your paid preparer) to discuss this tax return with the IRS? see instructions. There is a YES box to check for that somewhere, probably the 1040 worksheet and it should allow you to enter another name.

If it's just you, the NO box auto-fills, which tells the IRS, the preparer is the 3rd Party Designee.