How to add a username to a Facebook page

video1.0<iframe src="https://www.loom.com/embed/11140888445c402480044584fa639bbb" frameborder="0" width="1482" height="1111" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>11111482Loomhttps://www.loom.com11111482https://cdn.loom.com/sessions/thumbnails/11140888445c402480044584fa639bbb-ce626934a916c524.gif153.0309How to add a username to a Facebook page