While Sathya is correct, there may be an option you would like to investigate.
Specifically, the policy "Allow Automatic Updates immediate installation".
This will download and install definition updates for Windows Defender automatically. As the description indicates it will also download other updates so it may or may not work for your situation.
Allow Automatic Updates immediate installation
This policy is only applicable when AU configuration options 3 or 4 are applied and this policy is enabled. This policy only allows for the immediate installation of minor updates. This policy setting specifies whether Automatic Updates should automatically install certain updates that neither interrupt Windows services nor restart Windows. If this policy is enabled, AU will immediately install these updates once they are downloaded and ready to install. If enabled this policy will override notify for install or scheduled installation configuration for these types of updates. If this policy is not configured and the AU configuration option is set to 4, the minor update will still immediately install. However, if you disable this policy after enabling when AU options are set to 4, the minor update will no longer immediately install.