Hi George,
Thanks for taking the time to submit this idea.

Additional validation was added to the TLD field on the "Use own domain" option back in 2019 based on feedback from partners that they were experiencing significant support overhead dealing with customers entering invalid values in this field. It's also generally advisable to validate field data from a security standpoint.

As such, we do require the TLD field to be a recognised TLD, this can either be one of the 200+ TLDs WHMCS supports out of the box, any you add yourself, and any domains with pricing configured: https://docs.whmcs.com/troubleshooting/troubleshoot-domains/domain-availability-errors/#troubleshooting

Admins can continue to place orders for any Domain value they wish, including TLDs not included in any of the aforementioned places. So in those situations, clients can contact your Sales team and they can place the order on the client's behalf. To make this easier, you might consider overriding the ordererrordomaininvalid language string with a custom message inviting clients to contact your Sales team, even including a contact link in the message:

$_LANG['ordererrordomaininvalid'] = "To place an order with this domain please <a href=\"contact.php\">contact us</a>";

https://developers.whmcs.com/languages/overrides/