Protect your links: a password is asked before taking the visitor to the destination.
Protect specific links with a password. Before redirecting, the visitor must enter it; without it, they don't reach the destination.
When someone opens a protected link, they see a page asking for the password.
If correct, they continue to the destination and it's remembered for a while (cookie), so it isn't asked on every click. If wrong, they don't pass.
From Add-ons, click Install on "Password links".
When creating or editing a link, the "Link password" field appears. Type it and save.
To change it, type a new one. To remove it, check "Remove the password".