It's awesome of you to reach out for advice about this. Kudos on your initiative and thoughtfulness. Playing the role of the wife, I'd say a short message would be great, though I agree w a previous comment, and add that it's not your job, especially at this early stage, to manage the other partner. It's thr job of your potential boyfriend's to manage his partner.
Having said that, I recently had the first experience of being introduced to a possible future gf. It was done in a poly-friendly social setting. That way my husband could spend some time w her while I socialized w others, then I could spend some time w her and w them. It worked out great.
As a final thought, this early on I would take offense if you suggested too much of a commonality between us. But that might just be me.