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":""},"content":"bXkgdXBkYXRlZCBmaWxlIGNvbnRlbnRz","sha":"329688480d39049927147c162b9d2deaf885005f"  
 **"branch" : "NewFeature"** }

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

Tried this as well"NewFeature"

Did not help.

Appericate your help


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