Unity Cloud Build requires a username and password to connect to your Mercurial repository (see www.mercurial-scm.org). This applies to Mercurial repositories hosted on Bitbucket or elsewhere.
In your Mercurial server, create a user just for Unity Cloud Build, along with a secure password. If your Mercurial host supports it, make this a read-only user account.
When you configure your project in Unity Cloud Build, you are prompted to specify which branch to build from.
You may need to define which folder contains your Unity project; specifically the Assets and ProjectSettings folders. This is usually the root folder in your repository: If this is the case, you can leave this field blank. However, if your Unity project lives in a subdirectory, you need to enter the path to the Assets and ProjectSettings folders during setup of a build target in Unity Cloud Build. It looks something like: NewGameProject/Src/UnityProject/ (please note that this is an example only).