Setting up and managing device configurations using Intune
Are you using the Templates -> Preferences type config profile?
That should be keys and values only. You need to strip out the plist wrapping and upload only that part. You file should look like this:
<key>ExtensionInstallForcelist</key>
<array>
<string>abcdefghijklmnopqrstuvwxyzabcdef;https://clients2.google.com/service/update2/crx</string>
</array>