Using EpiFind nuget package to index, how do I set the ID

I have an object like below. I have defined the _id field but I do NOT get the value I passed set as the ID.

How do I force it to use MY id that I pass it?

    public  class MarketReportItem : ISearchContent
    {
	    public string _index { get; set; }
        public string _type { get; set; }
        public string DocumentGuid { get; set; }
        public string FileName { get; set; }
        public string _Type { get; set; }
        public string _id { get; set; }