Go to Datto Downloads Page
Download the Endpoint Backup for PC's
Run installer on the computer you are trying to add to your portal
Once the installer finishes you will be promtped to Log into the Datto Partner Portal
it will auto navigate to:
Status → Endpoint Backup Status → Register Agent (top-right of the page).
In the Register Datto Endpoint Backup Agent window:
Once all fields are completed, click Register.
Finally, Confirm that the process worked and you can see the new agent in the Datto portal
Leave the Endpoint on untill it finishes it first backup
Go to Datto Downloads Page
Download the Endpoint Backup for PC's the MSI installer
In the Partner Center go to "Status" in the Top Navigation Pane

In the Drop Down Select "Endpoint Backup for PC's Status"

Select the Company that you would like to install the agent for
Click on Create RMM Templete

This will generate a Token for you to use in the Silent Install
Transfer the MSI file to the endpoint you would like to install on in this example I have sent to C:\Temp
Change Directory to C:|Temp
cd C:\Temp

msiexec /i DattoEssentialsx64.msi /qn /norestart REGTOKEN="<token>"