I am using Visual Studio Team Services 2015 and I have multiple branches of the source code. I did not have any trouble in branching out the source code, however, I would like to implement Continuous Integration so as to automate the build process. However, I am not able to create a build definition for specific branch. Below are the links to the screenshots. You can see that I have Main branch (trunk/working copy) and version 1.0 branch (released version). However, the branch selection is not available while creating build definition.
What you are experiencing is expected. There is not an option to select a branch for a TFVC team project. You can map the branch you want to build under Repository tab: