Currently, Bamboo Cloud (OnDemand) instances are privisioned with the 'Atlassian Bamboo Stash Repository Plugin' disabled. Please have this plugin enabled so users can link to a Stash repository using the 'Administration' menu from Bamboo.

According to the documentation, this should be possible. Currently users are unable to select 'Stash' from the Repository host field when trying to link a Stash repo with Bamboo. There's a request to have the documentation edited, here: https://jira.atlassian.com/browse/BAM-15177.

Workaround

Users should still be able to connect to a Stash repo from Bamboo OnDemand, but not this way. Instead, users should go to the clone tab from their Stash repo, get the http or https clone address, go back to Bamboo and add a repo (e.g. type: Git) using that address. When adding a repo, under the Advanced options, there's a "Web repository" option, from there users can select Stash. It will checkout from stash the same as it would from any other repo, but won't have the same features provided by the plugin.