Basically what I'm asking is can you make a comment and do something similar to how on Twitter you can put @<desired person's username> anywhere in any post/comment/reply you make to make sure they notified about said post/comment/reply, if e621 has any feature like that please tell me how to do it.

If you include a username anywhere in a comment, that comment will show up in that user's "Search for myself in comments" page. They're not notified, though; they have to check the page manually.

If you send a user a message (dmail), they'll be notified.

But note that most artists don't have accounts here, and even when they do, their username doesn't necessarily match the name under which they release their art.