XF 1.5 Custom True/False Field

My old forum had a custom boolean field.
What is the best way to duplicate this in xenForo with a custom field?

I'd like to have the field titled, "Use Current Location" with a single checkbox after it. My existing data is stored as either 0 (false) or -1 (true).

The xenForo custom field editor is forcing me to add possible values to the field - as if it were a drop-down list. How can I set this field up to store the data as a simple boolean?

Thank you,