For an Echo notification to be sent, a message just needs to contain a link to the user page, but there are also templates for making this easier, such as https://en.wikipedia.org/wiki/Template:U and also https://en.wikipedia.org/wiki/Template:Reply_to which prefixes the username with an '@' and suffixes with a ':'.
We should see which conventions exist on various wikis for replying/mentioning, so we can work out which cases we should support.
We should also consider if wikis would be open to changing their conventions if we provide betters tools for replying/mentioning.