We have not improved LT GetN for 5 years, so we can add this in 2021.
I have added ORG-22180
Just curious that you posted this on OC forum, and you can certainly have a slider in OC like this
GETN_BOX( tr );
GETN_SLIDER(pos, "From", 0, "0|255|255")
if( GetNBox( tr ))
printf("result=%d\n", tr.pos.nVal );