This can be done by a custom hook using the ClientAreaFooterOutput and the ShoppingCartValidateCheckout hook points.

For more information on using this hook points please review the developer's documentation links here:
https://developers.whmcs.com/hooks-reference/output/#clientareafooteroutput
https://developers.whmcs.com/hooks-reference/shopping-cart/#shoppingcartvalidatecheckout

These community posts may help you with accomplishing that they enter the company name when registering.
https://whmcs.community/topic/267561-making-a-required-field-in-registration/
https://whmcs.community/topic/294989-customisations-on-the-registration-form/