Welcome to the community!
I did some search and found that you'll need to use 'memberID' when you want to mention someone from the Slack webhook.
For example, I have a user name Syahrul with 'memberID' of 'U88271KS', I'll pass this in the message as the following example:
script: - echo "Notify someone in Slack" - pipe: atlassian/slack-notify:2.0.0 variables: WEBHOOK_URL: '<Webhook>' MESSAGE: 'Hey <@U88271KS>'
This create @Syahrul notification in my Slack test channel as expected. You can check the guide here to find the Slack 'memberID'.
If you looking into formatting your Slack message, I would suggest you check the Slack API formatting page below:
That said, you can use *BOLD* in your message to bold a text message.