Promotion without a page is noise
The instinct is to post everywhere first and build the sign-up page while people wait. That order leaks registrations. Someone sees the post on Tuesday, clicks on Thursday, and there is still nothing but a mail thread asking them to email their name. Every day between interest and a working page is a lost attendee.
So the page comes first. A registration page that says what the session covers, who runs it, when it starts, and what happens after someone signs up. Only once it works does the link go out.
The same link does all your promotion: the post, the email signature, the WhatsApp group, the invitation email. One place to update when the time changes, one list where every name lands, and no version of the truth living in three threads.