Add domain to protonmail account

I have a mail account in protonmail.com with possibility to add my domain.

I have domain paid in ovh which I use to access my web page (not in ovh) and I would like to bind it with my mail account on protonmail servers.

For that I need to do the following in my domain config that will allow protonmail to reach for the domain:
_For security reasons, we need to verify that you are the owner of your domain. Please add the following code into your DNS record. Do not remove it even after successful verification. This can typically be done in the control panel of your domain name registrar._

And for that I have this entry
TXT @ protonmail-verification=00123123123...
This is probably the first step to do that before I set up MX and so on.

I copy-pasted it and saved into DNS zone when I have also my www server configured and tried to verify with protonmail and it fails.

What am I missing?