Correct way of writing my VAT number to make a purchase through my business?

Dear Forum and Customer Care team,

I'm currently planning to make a purchase of a relatively expensive monitor. This purchase is for my business and logically I will want to get a tax-writeoff for it. 

I live in Spain and my company's national (and hypothetical) VAT number is: E-12345678. My business is only registered nationally within Spain, not as one that operates internationally in the EU (or atleast I don't think?).

So I'm purchasing this product over the spanish Dell website, and when I go to write down my VAT in the according field during the checkout process, it tells me the format is wrong. I have been in contact with support about this and they suggest to ad "ES" (presumably for Spain?) in front of my VAT. This does indeed get rid of the error/warning message and let's me carry on with the checkout. However, I am anxious that this would mean that my VAT is wrong on the invoice and I won't be able to get the tax writeoff, as it deviates from "E12345678" and results in "ESE12345678".

Any business/tax guru that may clarify if adding ES would result in an inability to get taxes returned?

Thank you very much in advance!