Automatically Fetch Customer GST Number in Quotation

I have customized the Quotation Doctype by adding a field for the Customer GST Number. Additionally, I have customized the Customer Doctype and added a GST Number field to store this information.

Now, I want the GST Number to be automatically fetched in the Quotation when I select a Customer. How can I achieve this?

Any guidance examples would be greatly appreciated!

Thanks in advance.