If you are loading the already loaded table, you RESIDENT function.
LOAD FIELD1, FIELD2, FIELD3, FIELD4 FROM SOURCE ;
LOAD DISTINCT FIELD1 Resident MAINTABLE ;
There is another option in INLINE to load from Field values, but for this, we already reload the application.
In the Script Editor, Insert --> Load Statement --> Click Load Inline
then INLINE wizard will open, then Goto Tools --> Document Data
Then you can see the List of the fields and select one filed and clicl All values and click ok.