This one's easy: you don't have to!
FICA (the acronym for Federal Insurance Contributions Act) is deducted from each W-2 employee's paycheck to cover contributions to Medicare and Social Security. You can see how they are reported on the W-2 here. Because this is a payroll tax and not technically part of your income tax, you don't enter anything related to FICA in Holistiplan and payroll (or FICA) taxes will only show up in Holistiplan when they show up on the tax return.
That will occur when if a taxpayer will have excess Medicare tax withheld of 0.9% (as reported on Form 8959) on wages in excess of $200k for single filers, $250k for married filing jointly filers, or $125k for married filing separately filers. In that case, you may wish to enter the amount of that withholding in the "Other" row after expanding the Withholding section at the bottom of the Scenario Analysis screen as shown below.
The other exception applies if there is Self-Employment (SE) income. In that case, those Social Security and Medicare taxes will show up in the form of SE Taxes on Schedule SE, and will be reported on Schedule 2. Aside from these two exceptions, FICA will not show up in Holistiplan, so you do not need to account for it.