Hi
After you have commence to receive the pension(s), you will need to download and print off and complete the DT individual UK / USA form at
Double Taxation: UK-USA (SI 2002 number 2848) (form US-Individual 2002).
On the form you must declare all the pensions including state pension. The completing and signed form will need to be sent to the IRS, for a fee.
You will need to find that address to send it to. The IRS will validate the form and send it directly to HMRC.
With the validated form, HMRC can ensure that no tax is deducted and have any tax deducted, refunded to you.
If you have not commenced receipt of the pension, you cannot put it on the form.
You may have to complete this form more than once, if your pensions commence at different times.
Thank you