Bamboo provides support for the following forward proxy parameters:

http.proxyHost
http.proxyPort
https.proxyHost
https.proxyPort

and not 

http.proxyUser
http.proxyPassword

for all types of outgoing HTTP requests. This prevents some Bamboo features from working in enterprise settings where forward proxy authentication is enforced for all outgoing requests.

Workaround
For interaction with external repository providers that use HTTP/S as the communication protocol (such as bitbucket.org and github.com) a possible workaround is to:

1. Configure the repository entry in the Bamboo Linked repository as type Git (instead of Bitbucket.org or Github).
2. Configure the git client that Bamboo is using as the Git capability to use a forward proxy. For example run the following commands as the same user that runs Bamboo on the Bamboo machine:

   $ git config --global http.proxy http://<username>:<password>@<forward_proxy_hostname>:<forward_proxy_port>
   $ git config --global http.proxyAuthMethod basic
   

   where you replace the <username>, <password>, <forward_proxy_hostname>, <forward_proxy_port> with the actual values. In the example above the basic authentication is used but you can find more options to configure the forward proxy related parameters at https://git-scm.com/docs/git-config The configurations wwould be saved under ~/.gitconfig

3. Run your plans.