How do I search by fields? Something isn't working

I am using EpiFind, the version is 12.5.3 in NuGet.

I want to search by a few fields in the JSON below. I am trying this code, but I don't get the right results back. I don't understand why.

Snippet

var searchResult = _searchClient.Search<RemoteHostedContentItem>()    .For(filename).InField(x => x.SearchFilename).GetResult();

I want to search by

SearchSubsection

SearchFilename

But I don't know how to.

{"SearchSubsection$$string": "S-Reliability Coordination","SearchTypeName$$string": "Document","SearchFilename$$string": "RTVSA_Summary_071718_1509.html","ContentTypeName": "Hypertext Markup Language","SearchHitUrl$$string": "http://localhost:17104/api/extranetdocuments/getbyname/RTVSA_Summary_071718_1509.html","_TypeShortName": "RemoteHostedContentItem","accesstype": ["Visitors-RA"
    ],"rtvsareportanalysisfile": "Case Summary","guid": "42b1048e-8995-4e89-804d-caafb55cdb3c","SearchSection$$string": "Documents","_type": "MisoWeb_Models_Find_RemoteHostedContentItem","Files": {"original": "http://localhost:17104/api/extranetdocuments/getbyname/RTVSA_Summary_071718_1509.html"
    },"_id": "Extranet_RTVSA-448","RolesWithReadAccess$$string": ["Visitors-RA"
    ],"_TypeDisplayName": "Remote Hosted Content Item","FileName": "RTVSA_Summary_071718_1509.html","CommitteeDocType": "test","CreatedBy": "MISO.C2.RTVSA","$type": "MisoWeb.Models.Find.RemoteHostedContentItem, MisoWeb","UpdatedBy": "MISO.C2.RTVSA","DocumentType": "S-Reliability Coordination","SearchTitle$$string": "RTVSA_Summary_071718_1509.html","rctype": "RTVSA","ContentType": "text/html"
}