To do this, you'll first need to log into Windscribe.com, and click on the ROBERT link under Account Overview. (illustrated in the attached screenshot)
Once there, you can choose to use one of our curated blocklists, or you can whitelist, block, or spoof any domain by visiting the Custom Rules section. (illustrated in the attached screenshot).
Once you've made the required changes, be sure to click yes on the "Apply to all connected devices immediately" dialogue that pops up.
Here is a quick link to this page: https://windscribe.com/myaccount#robertrules