If you want to embed a promo code in an event registration link all you have to do is add the following to the end of the event URL:
?promo_code={promotion-code-here}
A sample full URL would look like this:
https://engage.secure.nonprofitsoapbox.com/sample-event?promo_code=1dollaroff
* Please note that in order for this to work you have to setup the promo code for your event first. Not sure how to do that? Check out this other knowledge base article: Creating Promo Codes for your Soapbox Event