Generally you must set an Event Partner before you add people to the Program.
Not sure what your exact issue is, but you would have to set the Leads to Not in Program, then set the ON24 connection, then send the leads back through Registered Status to get them to sync to ON24.
On24 is a it tetchy, so please carefully read the docs and decide on how to manage the emails and reg process (in Marketo vs. partially in ON24).
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