Per Maryland Form 502 Instructions, on page 4:

7. Enter the name of your county on the MARYLAND COUNTY line. If you lived in Baltimore City, leave the MARYLAND COUNTY line blank.

In order to correctly calculate the Baltimore City Tax Rate, the County field in the address section must be left blank. In order to do this, please click No County in the County field.

To enter the information in TaxAct:

1. From within your TaxAct return (Online or Desktop), click State to expand, then click Maryland (or MD). On smaller devices, click in the upper left-hand corner, then click State.
2. Click Basic Information in the Maryland Quick Q&A Topics menu to expand, then click County and City.
3. Continue with the interview process to enter all of the appropriate information.
4. On the screen titled Maryland - County, select "No County" from the bottom of the County drop-down, then click Continue.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.