Using the TurboTax desktop editions -
Click on Personal Info
Click on Continue
Edit your personal information
On the screen Do Any of These Apply to (your name)
Check the box Another taxpayer can claim (your name) on their tax return
Using the TurboTax online editions -
Click on My Info on the left side of the online program screen
Edit your personal information
On the screen Let's check for some other situations
Check the box Another taxpayer can claim me as a dependent on their tax return