You have no data attached to your request; you're loading everything as a header. That could easily result in the error that you're seeing.
Items that are meant to be part of the request payload, not the headers, should be added via:
request.AddParameter("text", "Message Schedule");
request.AddParameter("socialProfileIds", "xxxxxxxx");
request.AddParameter("scheduledSendTime", "2018-07-27 17:30:00");
request.AddParameter("emailNotification", "false");
It also looks like the RestSharp API expects authentication to be handled differently, and that might be interfering with your testing.
Read the docs:
Home · restsharp/RestSharp Wiki · GitHub[
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