HTML5 extra attributes in registration page

[marvays]

Hi.
I'm solving a problem. I use Rupostel OPC where I have HTML5 extra attributes on the phone field. I need to set the same during registration. It is not among the custom fields in the Virtuemart configuration (too bad)... it could be added there

But I can't find a php file in which I can set this when registering for a phone number.
Can you advise me please?

For the phone field I need to set:

Code Select Expand

pattern="^\+420\s?\d{9}$"