Insurance Premiums for Retired Public Safety Officers. IRS instructions say enter reduced amount on Line 5b and write "PSO" next to that. How can I do that in Turbotax?

Turbo Tax will automatically do that if you enter the 1099R right.   After you enter the 1099R keep going.  One of the next screens will ask if you are Public Safety Officer.  If it doesn't work right then   delete the 1099R and try entering it again manually.

Enter a 1099R under
Federal Taxes on left
Wages & Income
Then scroll way down to Retirement Plans and Social Security
Then IRA, 401(k), Pension Plan Withdrawals (1099-R) – Click Start
If you are adding another 1099R there might be a Add button at the bottom of the list.

Don't type the bank name or try to import it.
On the Let Us Enter Your 1099 page go down to the bottom and pick Enter a different way

Did a search and found this post

https://ttlc.intuit.com/community/retirement/discussion/pso-deduction/00/3174912