Peter and Jan are both correct, depending on how you "redirect"
If you use the IIS Url Rewrite engine to redirect, or easier just set the domain to redirect to the main domain through your DNS, then you don't need a license since the redirect will occur before "Kentico" gets hit. This is the case that Peter is correct.
Jan is correct if you want Kentico itself to handle the redirect, as then Kentico needs to validate the domain (where it will need a license key). And you can obtain a domain alias for free after approval (following Jan's comment).
For simplicity though, if it's just to redirect, use Peter's.