Hi!
Check the livedocs for UIScrollBar component:
http://livedocs.adobe.com/flash/8/main/00004219.html
The example at setScrollTarget() should be enough for what you intend to do.
Another example. I have a MovieClip named content_mc which contains a text field named content_txt and an instance of the UIScrollBar named content_sb. The code goes something like:
content_mc.content_sb.setScrollTarget(content_mc.c ontent_txt);
content_mc.content_sb.setSize(16, content_mc.content_txt._height);
content_mc.content_sb._x=content_mc.content_txt._x +content_mc.content_txt._width+14;
content_mc.content_sb._y=content_mc.content_txt._y ;
content_mc.content_sb.lineScrollSize = 2;
content_mc.content_sb.pageScrollSize = 5;
You could easily add something like:
content_mc.content_txt._alpha=60;
content_mc.content_sb._alpha=80;
[Note: Those 14 pixels are there for padding between scrollbar and textbox]
Hope it helps