How do I tell you about an IOTA activity on my callsign?

You don't need to! All IOTA records are managed by the IOTA web site itself, and not by Club Log.

IOTA uses Club Log matches as part of confirming QSOs, but the IOTA award does not depend on Club Log to decide which IOTA (if any) the QSO qualifies for. Club Log does the matching, and the IOTA software chooses the island.

If you want to let the IOTA team know about an island activity, you could contact them directly.

If you're going on an IOTA expedition, please don't add callsigns like G7VJR/AN-123 (etc) to Club Log. This will cause your log not to match the DXers' logs when you upload them. Instead, use the callsign you use on the air, and allow IOTA to handle the separation of the QSOs for the islands and awards. Even though in the past various logs have been uploaded with IOTA add-ons after the callsign, this has proven to be a lot of trouble and is now blocked since IOTA can handle the log without.