class ActiveAdmin::Inputs::Filters::StringInput

def to_html

Else, build a search field with a companion dropdown to choose a search condition from.
If the filter method includes a search condition, build a normal string search field. 
def to_html
  if seems_searchable?
    input_wrapping do
      label_html <<
      builder.text_field(method, input_html_options)
    end
  else
    super # SearchMethodSelect#to_html
  end
end