I realized it was some different when I loaded local project to Visual Studio Online in Visual Studio 2015 compared with using Visual Studio 2013.
The following is the process of how to load a project onto VS online in VS 2015:
1: Create a local project in VS 2015 first;
2: There is a “Publish” button on the bottom right corner of VS 2015, click it:
3: Select Git option if you see pop up menu, then we can see the following screen, we choose “Publish Git Repo” under the "Publish to Visual Studio Team Services" section then:
4: You might be asked to input your Microsoft account, and then you get your existing VS online server list to let you select one, the Repository name is automatically set to your current application name;
Note: If you have already created a project on your VS online account, you should click “Advanced” to select the existing project first;
Then I just click the “Publish repository” button to submit our code:
Now your project should be on your VS Online account, you just login into your account and confirm the code has been already loaded.