I believe it is on the 941.

The employee retention credit is claimed against “applicable employment taxes.”

See link for more info. https://www.irs.gov/pub/irs-drop/n-21-49.pdf

You amend payroll (941X) and then amend the business return for the year(s) impacted, because this is Payroll, not Income taxes.

lots of people getting emails and daily calls about the ERC and asking their prepares. Befor you knock yourself out, check their eligibility… this may help or Google ERC guidelines or something like that… 

https://www.irs.gov/newsroom/employee-retention-credit-2020-vs-2021-comparison-chart

"lots of people getting emails and daily calls about the ERC"

I saw a big ad by some provider on TV! I don't recall whose ad or which channel, and I normally FFWD through the commercials, but this one caught my attention enough to see it was using the "get $26,000" to bait that hook.