When you choose this action for a validation rule, a field complement is created for a field.
Specify a name for the action. This is just a label to help you remember what the action does.
Specify which field is to have a field complement.
Type a value for the complement. Either type an expression yourself, or click Expression, which opens a dialog that can help you write the expression. The result of the expression must be a string or a numerical value. (If you want to write a simple text message, enclose it in quotation marks.)
Click OK.
Adding validation rules to forms