I'm using a custom form but in the field for the customer tax reg #, quickbooks automatically adds VAT No before the number and is not correct for my countryCan i change?

Thanks for bumping up again, tukan. I would like to help you out with this. 

It could be that we are using a different QuickBooks international version. That is why it is adding a prefix Vat No in our customer's Tax Registration No.

If that's the case, we can create a form and import it to QuickBooks. It's a way to correct and pre-fill the form of your customer. We will first need to turn Import style on from the QuickBooks Labs and import your .docx file Microsoft Word.

Let's follow the steps below to do so:

1. Go to the Gear.
2. Select QuickBooks Labs under Your Company.
3. Switch the Import Style ON. Then, Done. 

After that, in the Custom form styles, click the drop-down arrow in the New Style button and select Import style. Check out this article. It provides details and steps on how we can do it successfully.

We can go to this link: https://quickbooks.intuit.com/choose-country/ and choose the right account for you. Let's use the same login credentials to do it. 

Please let me know how it goes.  I would keep an eye on this to make sure that you 're getting the things you want in your book. I'll be around to respond as soon as I can. Take care!