Utilizing state/customState with passport-azure-ad

I’m having trouble figuring out the purpose of customState and if/how I can utilize it to pass data to the return url. Specifically I wish to route the user back to their original location after being signed in. I thought I could pass the original url to the parameter customState and have it returned back to me in the return url POST, but it appears to be encoded or perhaps replaced with a different value.