MYSQL Workbench Column Default/Expression
You are kind of mixing 2 different topics.


DEFAULT clause only accepts constants in MySQL. For example 0, -1, '', or '2000-01-01'. Workbench cannot override this limitation.
Virtual columns may be what you really need (or not). They have an automatic value determined by an SQL expression (no queries), but you cannot change that value. Check this article for more details.