Download the replication snapshot file using FTPS
Usually, SFTP is a specific port that you specify and the protocol sees the port and negotiates the encryption and shares the public key. It looks like you can use the @ftp_port argument in your publication to specify the appropriate sftp port (usually 22).

ftp technet article: https://technet.microsoft.com/en-us/library/ms146956(v=sql.105).aspx