How to get guid for custom entity?

Hello everyone!

Is is possible to write a coding to retrieve data from new_taxrate (custom entity) based on new_customertaxcode & new_producttaxcode (custom fields) in quotedetails?

My idea basically, I want to go new_taxrate (custom entity) and get guid of new_taxrate. Then send another URL to get percentage in new_taxrate. It is possible to go in CRM using Javascript?