How to test run a Bamboo deployment from a different branch?

Question

We are using Bamboo Deployment Projects configured in our bamboo.yml:

# ...
---
version: 2

deployment:
  name: MyDeployment
  source-plan: MYPLAN
environments:
 - Dev
Dev:
  tasks:
    - script: echo "hi"
# ...

Now suppose I create a git branch and I change the deployment task in the yml file. How do I create a deployment to test my new deployment task before I merge it?

If I push this branch, run the build for the plan branch, check that "bamboo specs execution successful" and create a release, then if I deploy the release still the deploy script from the master branch is used.

Yaml format: https://docs.atlassian.com/bamboo-specs-docs/9.4.0/specs.html?yaml#release-naming

Deployment projects: https://confluence.atlassian.com/bamboo/deployment-projects-338363438.html