//...
SourceStream := TStringStream.Create(UTF8Encode(strGraphQL));
//...
TIdHTTPAccess(IdHTTP).DoRequest(Method, URL, SourceStream, ResponseStream, []);

query locations {
  locations(first: 10) {
    edges {
      node {
        id
        name
      }
    }
  }
}

query post($filter: PostFilter) {
  queryPost(filter: $filter) {
    title
    text
    author {
      name
    }
  }
}

{
  "filter": {
    "title": {
      "eq": "First Post"
    }
  }
}

{
  "query": "...",
  "operationName": "...",
  "variables": { "myVariable": "someValue", ... }
}