FTPS and IPSEC -- What is the shared key?

[MisterProtocol]

I have a Sony A7 Mark IV with firmware 3.00.  I am trying out the security features.  So far I've managed to upload photos to the Sony cloud using the stored credentials method, which I believe uses SSH.  I'm now trying to exercise the FTPS/IPSEC features, which are older, and am having some trouble.  Has anyone ever gone through the exercise of installing a root certificate from a Certificate Authority, generated a certificate for use with an FTPS server, and transferred photos over the encrypted link?

So far I have configured a FileZilla FTP Server on MacOS Sonoma, and using the server's built-in ACME client, successfully generated and installed a certificate from Let's Encrypt.  I have also successfully installed the Let's Encrypt root certificate into the camera.

When I enable IPSEC on the camera, it requires a destination IP address (not a problem) and a "shared key" (problem!).  The process of generating a certificate from Let's Encrypt does not appear to generate or require an 8-to-20 character shared key string.

What is the certificate generation process that provides or creates such a string?  FTPS doesn't seem to work without it.