Just pressing the key combination inside the input box worked for me, although I had to make a few attempts before I found something that wasn't already used by another browser function.
Right now I use CTRL+] to ROT13 selected text, which in the LeetKey settings shows up as "Ctrl+VK_CONTEXT_MENU". It may be relevant that I use the US-International keyboard layout.
If I type Ctrl-] in the box, it just shows "Ctrl+". If I then press ] alone, "Ctrl+" disappears and it just says "VK_CONTEXT_MENU".
Every time I come across one, it annoys me. I have to copy the text, open a new tab to the ROT13 site, paste the text, and click the translate button.
Compare this to something like a collapsable spoiler button-box where you press a button and it expands and expands a box with the appropriate text underneath it. Even making a [spoiler][/spoiler] tag that gave a black background and equally black text would be better than the current ROT13 solution.
Was there actually a reason for doing things this way? If so, why not just include the ROT13 translation in the javascript that'd open and close the textbox? Comments? Criticism?