'Auto Generate Reference if HTML field left blank using MYSQL count of foreign key

I have a input field in a basic html form which the user can either add their own reference number, or they can leave this input blank. However I want it so that if the user leaves the field blank that the sql will generate a reference number based on a count of a foreign key in my mysql table.

INSERT INTO my_table (`reference`,)
VALUES (CASE {$inputValue}
            WHEN '' THEN sum(case when `fk` CONTAINS {$inputFK} then 1 else 0 end)
            ELSE {$inputValue}
        END)

When I've run the above code I get '#1111 - Invalid use of group function'

Is there a better way of doing this, or is there a way of fixing the above code?

htmlmysqlsql


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