Get attribute value from entities CRM online

Hi all, I use web service to get attributes, but always from the same entity where the page is when the web service is executed.

I use:  Xrm.Page.getAttribute("attribute_name").getValue();

But what I want to do is for example execute a web service in quote entity and get attributes from opportunity entity, or product quote, or any other entity ....

Anyone knows how can I do this? I´m using CRM 2016 online.

Thanks a lot,

Idoia