There is no ‘proof of owner’ or ‘document that I am the owner the domain name’.
The information about holders has been hidden on Whois since GDPR came into force 2018-05 for several different TLD’s. Certain information can be viewed via Whois, for example public information for .SE and .NU domain names is displayed via the Internet Foundations website. You can read about what is shown in Whois for the different domain names we offer here.
Sometimes it can be enough to verify control of the domain name with a TXT or CNAME record, alternative some actors can ask for copies of invoices. You will find all paid invoices in your customer zone at the bottom.
For some gTLD ngTLD domain names ( .com, .net etc. ) it may be possible to disclose the information in Whois. If you are interested, please contact us at registry@loopia.se with your customer number FAXX-XX-XX-XXXX the domain name and the wish to publish the contact information publicly in Whois and we will look into the possibilities for this.