Granting third-party apps access to iCloud Drive

I'm trying to get syncing going between vim apps on an iPad and a MacBook. (Actually, iVim on the iPad and MacVim on the MacBook.) No problem on the iPad side. iVim has access to iCloud Drive.

Not so on the MacBook side. MacVim is not on the list of apps in /system preferences/icloud/icloud drive/options.

Hoping there is a way to add apps to the list.

Any help appreciated.