Welcome to the Community. Reimbursement are non taxable pay back to an employee. Since the health insurance you're trying to reimburse is considered as taxable, you can use the Other Earnings instead. I'll show you how:
Click Workers tab on the left menu.
Choose the employee name under the Employees tab.
Click the Pencil icon beside Pay to edit.
In the How much do you pay this employee? section, click the Pencil icon.
Select Add additional pay types if you haven't selected any other pay types yet, or click the pencil icon if you have.
Select Even more ways to pay option, then choose Other Earnings.
You can also click the Add other earning type, then create a name for your pre-tax reimbursement.
You can now manually enter the amount when you create a paycheck for your employee.
I'm also adding help articles for your reference, read through: