On the forecast placeholder for that day, you can select the Zwift race and then ‘Pin’ it. When you Adapt Forecast, Xert will adjust the rest of your training around those events.
Check out this forum thread for step by step instructions:
For what it’s worth, I usually pin any planned activities for the week on Sunday before my training week begins, rather than pinning weeks (or months) of activities in advance. You might see if that approach works well for you.