1 - Reference the component
2 - Drag it to the screen, after the textarea and your save button
3 - Fill the inputs
TagList: The list of records that can be tagged.
TextVar: The text variable associated to the Textarea
TextareaId: The runtime Id of the Textarea widget (the widget needs to have a name so it can have an Id)
SaveButtonId: The runtime Id of the save button (the widget needs to have a name so it can have an Id)
4 - Add the handlers:
MentionUser: Triggered when a record is selected, here you should append to your Textvar the input of the event.
Send: This is triggered when your save button is clicked, here you can perform the save logic and you'll have access to the list of tagged records. (the destination of your save button should be a dummy action).