A custom installer is one where the client account and computer account information are already locked into the installer. A custom installer is useful in that it allows you to give out an installer which will be set up as the computer account you intend.
Generating a Custom Installer Link
Log into the management portal and go to the computer account you want to generate a custom installer link for.
Go to the Settings tab and click on the Create Installer link.
A pop-up window will appear which will give you a link to download the custom installer and an opportunity to email the custom installer link to an email address you choose.
Once the installer has been downloaded, use it to install the Backup Agent using the normal procedure.