You can specify the certificate on the TLS/SSL page:
https://winscp.net/eng/docs/ui_login_tls
But you first need to select a protocol that actually uses TLS/SSL – FTP or WebDAV – not SFTP. And enable encryption.

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Btw, WinSCP 5.13.3 is almost 5 years old. You really need to upgrade.

Hello, I wanted to ask something, since I've tried looking around for it and not found any luck in answering it already. Are SSL certificates still being supported in version 5.13.3? I saw that they were at one point, but they don't seem to be an option anymore, at least in Advanced Site Settings. I also read somewhere that they could be enabled, but I haven't seen anything definitive about that. I'd greatly appreciate the help.

Thanks in advance!