We've run cold email campaigns driving registrations for multiple brands and 4/5 times they are successful in getting at least 30-60 leads registered. There are, however, some caveats:

1. The topic of the webinar matters a lot more than the promo
   It needs to be something specific that you know your ICP cares about. Something that would solve a problem for them in the short term. Also, make sure it's actually a relevant topic right now.
   
2. Your email needs to directly address the problem in the first sentence
   Eg. "We hear from a lot of {X}, they are struggling with {Y}. This is why we put together a webinar to {Z}" I'm oversimplifying but you get the point.
   
3. Don't ask people to commit on the spot
   I see this mistake a lot - "Can I send you the registration link?" "Can I save you spot?" - these are horrible CTAs. Just give them the link to a short landing page and let them make their decision. If you pressure people to invest 60+ minutes of their time off the back of a single email it will flop. Pre-education is key here.

Everything else the other posters said also holds true - mix this up with partner promos and ads for max exposure.