To search across a custom field, you would need to add a custom IComputedIndexField
but if values shows up in the index with Luke then your field is indexed.
i think you can do that:
create a new class inherit from SearchResultItem, and add a new propertie to your class to get the value of your custom field example:
[IndexField("Name of the field in Luke")]
public string PropertieName{get;set;}
replace SearchResultItem in your Query by the Name of your new Class like this:
SearchContext.GetQueryable<MyNewClassName>().Where(x=>x.PropertieName==Value);