How to add filter 'fq' in drupal 8 search api solr?
I have reached out to drupalsupport IRC got an answer from there. By the time I have got the reply from them debugged the code founded a createFilterQuery function in SearchApiSolrBackend.php file

$solarium_query-&gt;createFilterQuery('your_indexed_filedname')-&gt;setQuery('your_indexed_filedname:"string_to_be_searched"');


Here your_indexed_filedname would be something like ss_5f_test_5f_1

Don't forget to put double quotes for string to be searched, otherwise it won't fetch you the result.