The form called must have this properties:

.WindowType = 1 && MODAL
.vRetVal = v && where v is a constant with the same data type of the value to return

If your form receive parameters,
In the INIT Event:
LParameters vParam1, vParam2, ... ,vParamN

To return a value,
In the Unload Event:
Return This.vRetVal

And... in some place (method or event)
Thisform.vRetVal = vValueToReturn

Good Luck
Esteban