For the portal to be enabled, you need to modify your SPX Template.
Email Protection... SPX Encryption... SPX Templates (tab)
At the bottom, expand "SPX Portal Settings" and check the box for "Enable SPX reply portal".
I am confused by your question about magic words because I thought you had already done this.
Email Protection... SMTP... Data Protection (tab)...
1) Data Protection Policy.
Ensure that "action on match" is "Use SPX Encryption"
2) Sophos Content Control Rules (optional)
There are a lot of rules here. Use the two pull-down fields to display different sections of the list(s), and choose the ones that you want
3) Custom Rules (optional)
Use Regular Expressions to search the message. It probably searches the raw content, which includes all of the headers and HTML tags, so beware of configuring false positives.
Also on
Email Protection... SPX Encryption... SPX Configuration (tab)
"Prefer SPX" checkbox only applies if you are using SPX and one of the other supported methods (S/MIME or PGP). Those require cooperating senders and receivers, so if both apply, I would avoid SPX, so I suggest leaving the box unchecked. But you probably are not using either of them, so the setting does not matter.