Rename sequential numbers at start of filename?

I can't figure how to rename by sequential numbering at start of a file name, rather than at the end as the built-in option in Directory Opus' rename dialog.
Maybe it's harder than I think.

This is an example of what I would like to achieve:
Old name New name
abc.jpg 1-abc.jpg
def.jpg 2-dep.jpg
ghi.jpg 3-ghi.jpg

Will I need a script rather than some simple expression in the 'New name' box?
Thank you in advance for any help.

Put [#] in the new name to tell Opus where the numbers should go, like this:

Thank you very much, leo. I've pounded on this for a while.
In my research I had noticed that [#] seemed to indicated sequential numbering, but I never realized the checkbox had to be checked for it to activate.
I'm a noob at this.
Is there a list somewhere of these modifiers (like [#]) that are valid when the checkboxes are checked?
The manual and help file seem somewhat sparse, maybe it's just me.
I guess regular expression syntax can be applied sometimes also, like ^,$, or .
Anyway, thanks again.

Using [#] to place the number is in the manual, see here:

In the Appendix section at the back there are pages on the Pattern Matching and Regular Expression syntax which Opus supports.