It's best to download the file as a PDF and upload it directly in the room as a file. Keep in mind, animations and embedded videos will not play. I would suggest downloading them from your file and uploading and playing them as MP4s in the room.
But, you can also choose to share directly from the Keynote application via Screen Share. You will need to use the screen share feature within the webinar room. This feature is only accessible to admins and presenters. Keep in mind animations and embedded videos will most likely not play successfully as the frame rate when screen sharing is slower than normal video playback.
Make sure to open the Keynote presentation on your desktop prior to sharing your screen.
Once you have your opened your Keynote Presentation, you will need to edit the view to prevent attendees from seeing your notes and and other layout settings.
From the Keynote navigation up top, select View.
Then check Slide Only.
Then choose Inspector and Hide Inspector.
Lastly, click Enter Full Screen.
Once you've edited the view of the presentation, just share the presentation via screen share.
Click the Screen Button within the top nav of the webinar room.
Then Choose Application Window.
Then select your Keynote File from your desktop.
And begin sharing it.