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

Click Create

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