Although best practice is to upload your 16x9 slide deck 1-2 days in advance of the live start time, you can upload the file to the player at any time.
If you are incorporating screen share in your presentation, you can add your screen share up to 30 minutes in advance.
You can dial into the presenter phone line up to 15 minutes before the scheduled start time. By this time, you should have your attachments uploaded but if you missed the deadline, you can always add them to the on-demand recording.
You can upload polling questions up to 5 minutes before the scheduled start time. By this time, you will be able to begin communicating with your audience via the presenter to audience pop-up message.
Once the live countdown hits 0, you will be able to start your presentation. The presentation will automatically end 4 minutes after the scheduled duration, so make sure to wrap up your presentation in a timely manner and hit "end presenting" when you are finished.
Be fully prepared for your webinar with this cheat sheet that gives an overview of the various cut-off times in the Pro webinar player.