I had a little bit of time, so thought I'd take a review of this..
I was also unable to get it to work, even when temporarily disabling firewalls at both ends, adding port 2304, enabling PAM authentication etc. I believe additional ports may need to be open.
Having seen the GUI, I wouldn't recommend using it in any case. Authentication should be done using keys, especially for a root account. Also, I'm not convinced that the API token needs full system access, even though I tried with both restricted & full.
For completeness, I tested the method that I had previously used: cPanel Account Restore.
Remote copy your backup files to the CWP /home directory, then import works fine.
You'll need to define packages to suit the imported accounts - easiest to do that first.
Remember to open port 2304 in the firewall (unsure as to why, as it's a local connection.)
Example remote copy (you should be using ssh keys to authenticate)
scp -P 12345 root@whm.server.com:/backup/2020-01-31/accounts/cpaneluser.tar.gz /home/cpaneluser.tar.gz
The GUI is self-explanatory now.