This feature is suddenly critical worldwide, as Google has suddenly declined to support backing up WhatsApp messages and pictures. So it's critical for my work and personal records.
NB this question was raised before indicating it's currently not possible. The thread has been closed for futher comments.
https://www.dropboxforum.com/t5/Create-upload-and-share/How-to-enable-specific-folder-auto-backup-on-my-Android-device/m-p/725254#M76683
Please keep this thread open so that others can also convince you it's necessary.