The first thing you will need to upload a file to ICloud is the entitlement: https://learn.microsoft.com/en-us/dotnet/maui/ios/entitlements?view=net-maui-8.0#icloud

This contains the following xml for the entitlement:

<key>com.apple.developer.icloud-container-identifiers</key>
<array>
  <string>iCloud.com.companyname.test</string>
</array>
<key>com.apple.developer.ubiquity-kvstore-identifier</key>
<string>$(AppIdentifierPrefix)$(CFBundleIdentifier)</string>

While the following docs are for xamarin, they may be helpful in getting the correct response, as the following Microsoft vendor states: https://learn.microsoft.com/en-us/answers/questions/1665676/not-able-to-perform-read-and-write-operation-in-ic https://learn.microsoft.com/en-us/previous-versions/xamarin/ios/data-cloud/introduction-to-icloud#finding-and-opening-icloud-documents