Hello,
if the function is not in the Shortcuts list then it is not possible to assign a shortcut to it.
Since this is a very old thread, it is quite possible that this shortcut option has been dropped along the way in older versions.
Thanks for the reply Olivia (notification went to spam). Decided to necro this related thread instead of starting a new one (glad to do so!)
If the (CTRL+K by default) shortcut for “insert link” has been removed (CTRL+K by default), please add it back! It’s a standard across nearly all web, email, and cemail client WYSIWYG editors. As basic as CTRL+B for bold, lots of users (including this one) expect it.