You really should read the fine manual.
Extract form the said manual:
Returning a Value From a Form
You can use forms throughout your application to allow users to specify a value.
To return a value from a form
Set the WindowType property of the form to 1 to make the form modal.
In the code associated with the Unload event of the form, include a RETURN command with the return value.
In the program or method that runs the form, include the TO keyword in the DO FORM command.
For example, if FindCustID is a modal form that returns a character value, the following line of code stores the return value to a variable named cCustID:
DO FORM FindCustID TO cCustID