There are a couple of ways of doing it. I use the second one below typically so I balance the account on each remittance.

It sounds like you would normally set up a purchase invoice for WCB that would have the first line showing as a dollar amount in the Amount column and the 2xxx liability account for WCB as the account.

The first option is to just do a General Journal entry at any time. Make sure you Debit the 2xxx liability account and Credit the 5xxx Expense account for the $21.

Or do the same thing you normally do on your next remittance. However, if you have $1021 in that liability account and you only owe $1000, then on the next remittance, you will have three line items.
First will be your normal remittance for the $1000. But if you look at the journal Report before you post, you will see it only reduces the 2xxx liability account by $1000 not $1021.

On the second line enter a description to let you know you are balancing the liability with the expense and enter the $21 using the same liability account.
Now you have a bill to WCB that is $21 higher than it should be. To correct this you need to add a third line with a -$21 to reduce the invoice total back to $1000.
In this third line, you use the 5xxx WCB Expense account.

The final journal entry Report (look before you post) will show a Debit to the 2xxx WCB Liability account for $1021 to bring that balance to zero, a Credit to Accounts Payable for $1000, a Credit to 5xxx WCB Expense for $21.

I personally have a recurring entry setup like this all the time because there is often a rounding error with percentages. When I don't need the extra two lines, I delete them from the Edit menu.

Thanks, Richard. That makes it clear.