ok in the end it works with @ [sys_id:user_name] message through the API 

exactly this way it is stored in the [live_mention] table 

if someone needs more detailed steps those are following:

• An '@' is typed in a mention-integrated text area
• Fire a request from the REST back end at "/now/v1/form/mention" (not a public api).
• As more characters are typed after the '@' step 2 is called with more data
• The REST service checks the permissions of the first 5 active users
• The REST service returns these first 5 users
• The mention popup displays the list of the first 5 users
• The user chooses one of these users
• An "at mention" readable code is added to the message
• The message is posted
• The message is parsed for the at mentioned text
• A live_message record is created
• A live_mention record is created
• The "Broadcast Chat APNS" (f110e602d710020023c84f80de610360) BR on insert for live_mention is fired
• new GlideLiveMessagePushNotifier().broadcastMentionFromRecord(current) with the new insert is called
• the at mention user is looked up
• the at mention user permissions are check
• a broadcast message is sent
• The Event manager queues the at mention
• An entry is made in the sysevent table
• low-level event batching happens
• "Activity Stream @Mention Email" or " Live Message Mention" email is sent