I want to implement the @tag user functionality in the conversation (comment section)

https://appian.rocks/2023/06/05/highlighting-stuff-in-text/

If you're thinking of the @tag style functionality that exists in, for example, this discussion forum, then there isn't an out-of-the-box method for this. You could so something like this (it's a bit basic but may give you an idea):

a!localVariables(
  local!users: {
    "A Aardvark",
    "B Bat",
    "C Cheetah",
    "D Dingo",
    "E Elephant"
  },
  {
    a!paragraphField(
      refreshAfter: "KEYPRESS",
      label: "Notes",
      value: ri!paragraphText,
      saveInto: { ri!paragraphText }
    ),
    a!buttonArrayLayout(
      align: "START",
      buttons: {
        a!buttonWidget(
          label: "Refesh to substitue @ for list of users",
          icon: "REFRESH",
          showWhen: fn!find("@", ri!paragraphText) > 0,
          saveInto: {
            a!save(
              ri!paragraphText,
              substitute(
                ri!paragraphText,
                "@",
                fn!joinarray(local!users, "|")
              )
            )
          }
        )
      }
    )
  }
)

When you say functionality, do you expect the tagged users to receive any emails or update regarding that mention?