We have added the new parameter lead_id to the PUT /mailbox/mailThreads/{id} endpoint. This parameter allows an email to be associated together with an existing lead.
Possible values for the parameter are the Lead’s unique ID which references the lead, e.g. dc775b7e-db49-444c-8a53-7ee785af1091 or null.
A little update: Our internal teams looked into it, and unfortunately, due to additional back-end complexity, lead_id in Activities is still in plans but has no ETA for the release. I’ve passed on your request for this, so hopefully, it can be prioritized sooner rather than later.