How do I ensure a custom text field is unique?

I’m creating an extension that will be a single line text field, and I’d like to ensure its value is unique. When setting a normal text field on a content type, I can have contentstack verify the field is unique before saving the entry by checking the box under properties > advanced > options > unique. When using a custom text field, I still see the “mandatory” and “non-localizable” options, but the unique option is gone. So how can I ensure the value entered is unique?