Naming a text box: I’m trying to copy a long text in transaction VL02N with copytext function, but the following statement didn't work: 

CopyText fromText="t1" toScreen="X[SPLITTER_CONTAINER]"

Since the text box doesn't have its own name in the screen elements you need to use another element nearby and a suitable offset. In VL02N you can use one of the buttons below the text box:

CopyText fromText="t1" toScreen="P[TP_FIRST]+(-2,0)"