Custom sort with search api
Go to 

Home  -> Administration  -> Configuration ->  Search and metadata  -> Search API

&amp; Edit your Index Search. Then your URL will be like below. In my case it's 

Content Search

/admin/config/search/search-api/index/content_search/

So I'm using

$data['search_api_index_content_search']['inspiration_first']

I think in ur case it will be

$data['search_api_index_search_content_index']['inspiration_first']

Please share your URL if didn't worked.