[CryWolf]

Ptereodactyl has his own SFTP integrated, and he downloads a clean server from steamcmd does not install addons unles has a script for it in main .EGG so go to game server Settings and there you will have a username connect to server SFTP port 2022 with that username and password form panel. and there you will have access to files and install sourcemod and addons.

Or you can direcly go trought files tab and upload via browser.