There are several ways,

You can declare a public var at startup and launch the form using
DO FORM myform NAME myvar LINKED (not recommended)

If you have an application object, add a custom property and launch the form using oApp.myproperty = createobject('myform') oApp.myproperty.show()
etc, etc OR, DO FORM and in the INIT of the form say oApp.myproperty = THIS

With either of the above, just call object.combo.requery().

You could also loop through the screen forms collection and look for the form name and call the combo requery that way.

You could also create your own forms registry in your application by using various methods available in FoxPro books, one authored by Jim Booth has a good example.