How to auto populate the fields values from one entity to another entity on selection of the custom button

Hi,

I'm working on requirement like , I created one custom button ADD VALUES button in Custom entity Sales Order.

In Sales Order I have 3 custom fields 1.Product Name, 2. Product Price and 3. Product Model.

now When fill above fields and  click on the custom button ADD VALUES I need to auto populate the above 3 fields values in Product Details Entity I do have same fields in Product Details entity.

Can some one please help me how achieve this requirement.

Thanks,

Rahul