Answer
Rather than hand-building the external-search URL and guessing at substitution variables, generate a genuinely correct URL directly from the client: navigate to the actual record you want to link to, right-click it, and use Send To > Mail Recipient as Link — this produces a correctly-formed URL for that exact record type in your mail client. Copy that generated URL as your template for the Event Action, and substitute the URL-encoded key value(s) using your event's own parameters in place of the specific key value shown in the generated example. This approach is simpler than manually constructing an external-search URL, and works correctly whether the user already has IFS open or needs to log in first.