To add a Directadmin server you first need to generate a login key.
To generate a key, login to Directadmin, and click on the user menu dropdown. (Top Right)
Select Login Keys
If you know the IP address of the server where Hosting Billing is hosted you can enter it in Allowed IPs
Leave blank if you want to be able to connect from any server with your login key
Scroll down and enter your Directadmin password in Current Password
Go to Servers in Hosting Billing, click Add Server