Can I run a proforma 2021 return in order to estimate withholding?

[Critter-3]

Have you tried using the IRS withholding estimation tool:

https://www.irs.gov/individuals/tax-withholding-estimator

Or have you tried using TurboTax's Tax caster tool:

https://turbotax.intuit.com/tax-tools/calculators/taxcaster/

[Mike9241]

there should be an estimated tax section for 2021 in your online Turbotax version.