To point your 1and1 domain to SITE123, you must have access to your domain management panel or be in contact with your domain provider. You will need to update 2 records in your domain's settings: an A-record and a CNAME record.
Please find below detailed instructions:
Please visit our support center at https://support.site123.com/ if you require more information.
Note - If you do not want to setup SSL, the Cname should point to global.site123.com