If you use the “config export” in your EVE-NG you might have noticed, that there are predefined passwords, that you have to use for this task.
If you want to change that, you can do this by modifying the files in /opt/unetlab/scripts/

You will find the vsrxng, vmx (both flavors), vQFX and others here – modify the “password” to your needs (Juniper, password1 and such).

Hope this helps you 🙂