I found a workaround in line 331 of sharing-sources.php, i don’t know how to do this through my theme though. Anyone know the answer?:
Original:
function sharing_twitter_via( $post ) {
// Allow themes to customize the via
return apply_filters( 'jetpack_sharing_twitter_via', '', $post->ID );
}
New (Twitter name where I have put “USERNAME” but no @ sign:
function sharing_twitter_via( $post ) {
// Allow themes to customize the via
return apply_filters( 'jetpack_sharing_twitter_via', "USERNAME", $post->ID );
}