Get the word out early, and increase racer turnout on race day.
If they pay in advance, they'll solidify their decision to actually show up. So early promotion using email and social media is the key. Luckily, your event's RaceReady registration page will have a custom URL which you can share in your outreach.
To get the URL for your event's signup page, log in to app.iraceready.com. From your main dashboard, find the event you're after and select the menu icon in its upper right-hand corner:
Select "Edit", then go to the "Review" tab for that event.
Scroll to the bottom of that page, and you'll find the URL pertaining to this event's registration. You can click "Copy URL" to copy it to your clipboard, and paste however the hell you want.