You can share prototypes by going to Plugins > Protowire > Share. You can either use Protowire.cloud or your own server. Your prototype will last for 2 days in the free version and 30 days in the pro version. To have it last longer, you just re-upload. The unique URL will be reserved for your prototype, so even if it's purged and you re-upload it, the URL won't change. Currently prototypes are publically accessible if the user knows the unique URL.
Currently the system supports the SFTP protocol when uploading to your own server. If you have SSH access to your server you will be able to use SFTP. The SSL checkbox refers to whether you have SSL enabled for your web server i.e. whether the web URL to access the prototype begins with https rather than http. All uploads to the server are encrypted regardless of whether you have SSL checked. You will need to create a directory where you store your prototypes and then each prototype will be placed in its own sub-directory.
Currently password protection of the prototype is not supported but you can configure your web server to use basic authentication either by changing the virtual host configuration or adding a .htaccess file to the directory if you are using the Apache web server.