I know this can be probably achieved via the workflow but wonder if it can be also done on the form when submitting??
How do we validate if the number/text within [PONumber] already exist in the List [Item Lists], colum [PONumber] and if it does then show error message. The form is loaded from the same list where it should validate against it, to check if the number/text exist in the column!
I was thinking something with lookup()
lookup("Item List", "ID", PONumber, "PONumber")Please advise?

