I want to create a query in Episerver Find where I can sort the results by a certain category name 

e.g If in normal free text search, I get 5 hits back from Find. If the user select category A in the filter on search page then I want to sort results by category A above all categories results like the example below even though category A items are lower in relevance.

Each page in the website is tagged by a category and the name of the category is an index in SeachCategories property. The name of the category is variable here.

item 1 => Category A

item 2 => Category A

item 3 => Category C

item 4 => Category E

item 4 => Category B

how this can be done?