I like Andreas generic code.
The methods ui.Dialog.setFieldValue
http://4js.com/online_documentation/fjs-fgl-manual-html/#c_fgl_ClassDialog_setFieldValue.html and ui.Dialog.getFieldValue
http://4js.com/online_documentation/fjs-fgl-manual-html/#c_fgl_ClassDialog_getFieldValue.html introduced in 3.00 could be used instead of the AUI tree reading and writing and would be safer in the long run.
If you don't want to go down the generic code path then DISPLAY will do what you were after ...
CONSTRUCT BY NAME where_clause ON field1, field2, field3
BEFORE CONSTRUCT
DISPLAY ">0" TO field1
CALL DIALOG.setFieldValue("field2", "A*")
AFTER CONSTRUCT
IF int_flag THEN
EXIT CONSTRUCT
END IF
DISPLAY "Field 1 = ", GET_FLDBUF(field1)
DISPLAY "Field 2 = ", DIALOG.getFieldValue("field2")
END CONSTRUCT
DISPLAY where_clause