How do we mention or tag someone in using Slack's Block Kit Builder?

I tried creating slack's block kit builder, the desired design were actually fine. But I don't know where to find a button so we can tag a someone on it.

I have here an attempt to tag a person named john but unfortunately it only generated a plain text, it did not notify the person nor became the same the design as expected (see image result below) view playground

{
    "blocks": [
        {
            "type": "section",
            "text": {
                "type": "mrkdwn",
                "text": "@juan \n\nis a mrkdwn section block"
            }
        },
        .....
    ]
}

Here is the result

I found the solution from the following link.

https://api.slack.com/reference/surfaces/formatting#mentioning-users

1. get the user's id
2. wrap it with a left and right arrow with an "@" symbol beside it. syntax <@userId>

usage:

{
    "blocks": [
        {
            "type": "section",
            "text": {
                "type": "mrkdwn",
                "text": "Hey <@802AWTP13BZ>, thanks for submitting your report!"
            }
        }
    ]
}

EDITED: Here's how to get a userID if you don't know:

1. Search for the person.
2. Click their profile picture.
3. Then, click 'Copy member ID' like in the picture.