Forgot to pay pa use sales tax what form do i use to pay it

If you haven't filed your PA return yet, then TurboTax will ask you this question during the interview process towards the end (look for the screen titled "Untaxed Out-of-State Purchases".

If you have filed already, then you have the option of either amending your PA tax return and including the use tax or you can file a use tax return directly online with PA using Online PA-1 Use Tax Return.  (You will need to create an account for this purpose)

For more information, see -

https://www.revenue.pa.gov/GeneralTaxInformation/Tax%20Types%20and%20Information/SUT/UseTax/Pages/Use%20Tax%20for%20Individuals.aspx