I can think of two ways to do this in QB. It will depend on the reporting and forms needed (provided?).

First, the easiest is to Change the payment transaction when you initiate Pay Liabilities. QB payroll, in the desktop, allows you to Preview the payroll transaction.

On the Expenses tab, enter a liability account and use a negative here for the amount you want to show is Held Back. Later, you pay that as a regular liability, not through Payroll. This will not change the 941/944, though. The form will show as if you paid in full.

Or;

Preview the liability check, where you can Zero Out or reduce any payroll item to leave that amount as owed and accrued, which will be the Employer SS item. For the Paydate (which is the liability date) range, then, you can let that specific Employer tax item ride by changing it to Zero or reducing it. You can use this for any delayed period.

For the following quarters/periods, you simply set the liability payment date as usual as the starting period date, and that means you again skipped the unpaid amount.

For the final quarter in 2021, you would select all the way back to the first Delayed paydate for Period, such as 3/13/2020-12/31/2021, and using a Payroll Quarterly report, compute the 50% from that skipped quarter, change the amount on the payment to not include the 50% you don't intend to pay. Then, in 2022, all things returning to typical, you won't do anything until 4th quarter, where you will again select for payroll liabilities the timeframe such as 3/13/2020-12/31/2022 for FICA will get the remaining Employer SS balance from March 2020.

By the way, you are in the wrong place on the internet. This is a peer user community for professional income tax preparers using Intuit's Tax Preparation programs. You are asking about QuickBooks and Intuit Payroll.

The online community for those tools is found at this link:

https://quickbooks.intuit.com/learn-support/us-quickbooks-community/misc/03/community-us