You mean @fooco.com? Or @foocousa.com? Or @fooco.xyz? @fooco.ai? @foocoltd.net? @foo.co.uk?
How would LinkedIn validate that your email domain belongs to the company you claim to work for?
Presumably because the official company page is registered under it?
What HelloNurse said, whoever it is that runs the company page on LinkedIn provides a list of domains that they consider theirs.
With a company-managed list of owned domains where real employees have their work email addresses (unrelated to website domains).