Greetings Gurus, this is a function I use very frequently to rename groups of files.
I actually use it as a hotkey and not a button for quicker access.
It also pads the numeric incrementor with zeros as necessary.
It's great for photos and videos which are arbitrarily named.
Try
Rename PATTERN=* TO="{dlgstring} " IGNOREEXT NUMBER=01
Did you forget to include the thing in your post? There’s only a screenshot of what it looks like.
Thanks for the simplicity in your response, lxp you are awesome with the one-liners bro! Does your solution also pad correctly for 100s and 1000s and so on? It seems hardwired for just a single added zero, can it be dynamic to account for any number of files?
Thanks Leo, It's updated now. Question for lxp, how do you figure out those one-liners?! I admire that...
Yes and yes.
@nodeselect
@set prefix={dlgstring}
Rename PATTERN=* TO="{$prefix} [#{=i = 0; while (Pow(10, i) <= selitems) {i++}; i=}]" NUMBER IGNOREEXT
Mostly by being lazy and happily letting Opus do the heavy lifting 
Wow, you refactored my code down to 2 lines, you inspire me...
The following code asks for a name for each file separately:
Rename PATTERN=* TO="{dlgstring} [#{=i = 0; while (Pow(10, i) <= selitems) {i++}; i=}]" NUMBER IGNOREEXT
Is there something like a command modifier that renames all selected files at once?
No, I don't think there is.
But if you really want a true one-liner and are willing to geek out a bit, use
=dlg = Dialog("s", "Enter Prefix"); i = 1; while (Pow(10, i) <= selitems) {i++}; return "Rename PATTERN=* TO=""" + dlg.text + " [#" + i + "]"" NUMBER IGNOREEXT"
Mind you, I didn't say it was pretty 
Thanks very much!