You should create pre_get_posts
filter and trigger in the $query->is_search()
to check if post_type
is set to author
and if it is then unset post_type
to posts or what evver you want and set author parametrs to $query
object.
related links:
https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts
https://codex.wordpress.org/Class_Reference/WP_Query#Author_Parameters
Or you should create another select, listing all users and give it a name of Author Parametrs
found in wp_query