GibHub API - How to edit a file on a feature branch its always defaulting to master?

Input given 

{"message":"my commit message","committer":{"name":"Scott Chacon","email":"schacon@gmail.com"},"content":"bXkgdXBkYXRlZCBmaWxlIGNvbnRlbnRz","sha":"329688480d39049927147c162b9d2deaf885005f"  
 **"branch" : "NewFeature"** }

Trying to edit a file on Newfeature branch but its getting edited on master?

https://github.ibm.com/api/v3/repos/project/repo/contents/test.txt

Tried this as well https://github.ibm.com/api/v3/repos/project/repo/contents/test.txt?ref="NewFeature"

Did not help.

Appericate your help

Thanks!

Does the branch already exist? If not, you may want to try creating it first.

I hope that helps!

Hello, I want to edit a file by GibHub API. Can you help me use it ? Thanks