First you go to your CF Dashboard > Security > Events and filter results based on the RayID mentioned in the block page. When you find it, the event details should contain the Rule ID for the rule that blocked the request. You can then use that string to search for the rule when creating an exception.
Once you click on the appropriate Ruleset, you’ll see a screen that lets you search for a Rule. After you paste the Rule ID you copied before, the rule will appear, with a checkbox next to it. Select it, then click Next.