(available in EditPad Pro only)

Select Prefix in the Block menu to prefix the selected block with one or more characters.   If you've made a selection that flows along with the text, the characters will be inserted at the very start of each line in the selection. If you've made a rectangular selection, the comment characters will be inserted on each line spanned by the selection, at the leftmost column in the selection. The inserted characters will become part of the selection.

See also

Block Menu
Block|Rectangular Selections
Block|Fill Columns