Active admin textbox/search functionality for selecting

 Add gem: 

gem 'activeadmin-searchable_select'

Import stylesheets and require javascripts:

// active_admin.scss
@import "active_admin/mixins";
@import "active_admin/base";

// active_admin.js
//= require active_admin/base

Add admin site in form field

f.input :account_id, as: :searchable_select, collection: AccountBlock::Account.all.map { |s| [s.profile.try(:name), s.id]}