I can’t find anything useful about this issue on Google or the burp suite documentation.
The app I want to use burp for, accepts only a password-username Login for Proxies.
Is there any way of setting a proxy username and password in the burp suite proxy instead of only a hostname and port?
On
Not sure if you still have the issue, but seems like "Platform Authentication settings" should be something you're looking for:
https://portswigger.net/support/configuring-ntlm-with-burp-suite
It is better to configure a VPN proxy for the system and use the burp. You cannot configure a credential for proxy in Burp.
Use Open VPN or Pulse or any other service and configure the proxy with the credential. Then use burp with loopback, and it should be fine.