How do I use the mandatory Title field if the member's title is unknown?

Title is a required field on a Member record when Address formally is selected. A title must be selected from the dropdown options.

It becomes an optional field when Address informally is selected.

You can then proceed to save the record with the Title field left blank. Do not guess or assume a title if the member has not provided one.

See also How to change the way Members are addressed in emails.