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 settings: A record and CNAME record.
Follow the steps:
Log in to your 1and1 account.
Click the Administration tab and then Domains.
Go to the domain you want to connect, click the arrow pointing down then Edit DNS Settings.
Select Other IP address.
In the IP address field, enter 188.8.131.52 and click Save.
New >> Create Subdomain.
Enter www >> OK.
Select the checkbox next to the www.
From the DNS menu, select Edit DNS Settings.
In the Alias field, enter ssl.site123.com and click OK.
Note: 1&1 Ionos is an external domain service and the steps of connecting your domain may change over time as they update their services.