see on GitHub

Commands: Querying: How to work with MoreLikeThis query?

To find similar or related documents use the MoreLikeThis method from the commands.

Syntax

MultiLoadResult MoreLikeThis(MoreLikeThisQuery query);
Parameters
query MoreLikeThisQuery A more like this query definition that will be executed
Return Value
MultiLoadResult Instance of MultiLoadResult containing query Results and Includes (if any).

Example I

// Search for similar documents to 'articles/1'
// using 'Articles/MoreLikeThis' index and search only field 'Body'
MultiLoadResult result = store
	.DatabaseCommands
	.MoreLikeThis(
		new MoreLikeThisQuery
			{
				IndexName = "Articles/MoreLikeThis",
				DocumentId = "articles/1",
				Fields = new[] { "Body" }
			});

Example II

// Search for similar documents to 'articles/1'
// using 'Articles/MoreLikeThis' index and search only field 'Body'
// where article category is 'IT'
MultiLoadResult result = store
	.DatabaseCommands
	.MoreLikeThis(
		new MoreLikeThisQuery
		{
			IndexName = "Articles/MoreLikeThis",
			DocumentId = "articles/1",
			Fields = new[] { "Body" },
			AdditionalQuery = "Category:IT"
		});