Form 4852 Substitute for W-2

In my last year's (2020) tax return, the CPA had filled in the form 4852 in reference to the pension I had received from my previous employer (HSBC bank) in United Kingdom. I have already paid taxes on my pension in United Kingdom. There was a note of an additional information on form 4852 stating that " Additionally, these distributions are exempt from US tax under the United States-United Kingdom Tax Treaty" 

I have completed all the information required for my tax return but, I did not come across form 4852 and guide instructions. Can you help me please?

Mukesh M. Desai.