You can not do this through modification form from your old registrar. You need to send the letter of authorization by fax or as attachment to e-mail message to us.

Letter of authorization is a document where the domain woner authorizes us to transfer and/or modify and/or delete the domain name.

If the domain name owner is a person, then the letter of authorization includes passport details of the owner and text authorizing to manage the domain.
If the domain name holder is an organization, then the letter of authorization should be on official letterhead paper and be signed by an officer of the organization.