When you are in the Storrito story editor you can tag / mention another Instagram user. Here a short tutorial:
First click on the "+ Add" button:
Then choose "@Mention":
There you enter the username, click on the search button and then you can click on "Add Usertag":
The usertag has been added to your story. You can also pick a different design for this usertag:
If you like to pick the font type and size, you can also add a normal text element and prefix it with an '@', this will automatically be converted to a clickable usertag. Pay attention that you spell the username correctly, otherwise your story post will fail.