You can get the fingerprint from the public key you download in the first line. Grab the file, and run this on it:
gpg --show-keys --with-fingerprint linux_signing_key.pub
The output for me is:
gpg --show-keys --with-fingerprint linux_signing_key.pub
pub rsa4096 2021-01-06 [SC] [expires: 2023-02-25]
CB63 144F 1BA3 1BC3 9E27 79A8 FEB6 023D C27A A466
uid Vivaldi Package Composer KEY07 <
[email protected]>
sub rsa4096 2021-01-06 [E] [expires: 2023-02-25]
which means
CB63 144F 1BA3 1BC3 9E27 79A8 FEB6 023D C27A A466
would be the fingerprint.
To be sure it's the right one, you need a secondary source of information. You can find it on https://help.vivaldi.com/desktop/install-update/obtaining-official-builds/ where it says
key ID that should end with โC27AA466โ.