First, I used the code of the following:

https://stackoverflow.com/questions/32300094/swift-write-save-move-a-document-file-to-icloud-drive

But, the copied file is not shown in iCloud Drive and
I found that iCloud capability must be checked in Xcode project's Capabilities tab.

https://developer.apple.com/library/content/documentation/General/Conceptual/iCloudDesignGuide/Chapters/iCloudFundametals.html#//apple_ref/doc/uid/TP40012094-CH6-SW24

But I also found that the Apple Developer Program membership is needed to develop iCloud apps. See the following page:

https://developer.apple.com/support/app-capabilities/

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And the other references to be useful for the future:

https://developer.apple.com/library/content/qa/qa1893/_index.html

https://developer.apple.com/library/content/documentation/General/Conceptual/iCloudDesignGuide/Chapters/iCloudFundametals.html#//apple_ref/doc/uid/TP40012094-CH6-SW24

https://developer.apple.com/library/content/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/iCloud/iCloud.html

https://stackoverflow.com/questions/43934427/ios-swift-3-copy-file-to-icloud-drive-programatically

http://www.techotopia.com/index.php/Using_iCloud_Storage_in_an_iOS_8_Application

maybe the following is about iCloud storage, not iCloud Drive

http://www.techotopia.com/index.php/Using_iCloud_Storage_in_an_iOS_8_Application