OPM 1099R

The box 2a is unknown.  Do I just enter the taxable amount as the amount in box 1 when filling in the form in turbo tax?  I know the full amount is taxable since my dad's been getting it forever.  I just hate to "modify" a form myself but it seems there is not another way to enter it into turbo tax except filling in the taxable amount as if the form itself had it completed.