All users:
$ProgressPreference = "SilentlyContinue" Invoke-WebRequest -Uri https://outlookclient.exclaimer.net/csua/Exclaimer.CloudSignatureUpdateAgent.Install.msi -OutFile C:\Users\Public\Downloads\Exclaimer.CloudSignatureUpdateAgent.Install.msi -UseBasicParsing msiexec /i "C:\Users\Public\Downloads\Exclaimer.CloudSignatureUpdateAgent.Install.msi" ALLUSERS=1
One user:
$ProgressPreference = "SilentlyContinue" Invoke-WebRequest -Uri https://outlookclient.exclaimer.net/csua/Exclaimer.CloudSignatureUpdateAgent.Install.msi -OutFile C:\Users\Public\Downloads\Exclaimer.CloudSignatureUpdateAgent.Install.msi -UseBasicParsing msiexec /i "C:\Users\Public\Downloads\Exclaimer.CloudSignatureUpdateAgent.Install.msi"
taskkill /f /im Exclaimer.CloudSignatureAgent.exe