I'm new to AWS Code build and trying to create a build configuration.
I've linked my bitbucket account with AWS. AWS is asking me to specify the branch from which I need to run the deployment.
I am trying to create a single pipeline for a build where I can select select the branch at the beginning before triggering any build.
Below is my configuration.

Can someone please help me understand how can I add a variable instead of branch name i.e. develop specified in above image?
Update : I tried following https://docs.aws.amazon.com/codepipeline/latest/userguide/actions-variables.html#actions-variables-examples-env-branchname but I'm getting below error.
When I try adding Manual Approval action group above source for specifying environment variable, I get this.
Source is always going to be a first stage. So I'm not understanding where should I specify these environment variables?
