How to search data of dynamic fields in sunpot solr?

Question

Created dynamic string field with below searchable do block in sunspot solr for rails app

dynamic_string :custom, stored: true do
  custom_fields.inject({}) do |hash, custom_field|
    hash.merge((custom_field.key) => custom_field.value)
  end
end

Solr save data inside document as

"custom:special_abc_ss": "abc"

How to query for this field on sunspot solr(i.e http://localhost:8982/solr/#/default/query) ?