@email@example.com the second icon beneath the toot, right of the arrow with a number (replies) and left of the star (like), the two bent arrows making a quadrangle is the 'boost' icon. Click it to boost. Best used on 'public' and not 'private' or 'outside public timeline' posts.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko