Need to Trigger bamboo Spec build only when pull request is merged with master branch

[Vetri]

actually i'm using bamboo spec in my build pipeline, i have a 2branches (master & release1)in my bitbucket repo,

now whenever we committed in to any branches that branch will automatically triggering the bamboo build separately , bt now i want to change that auto triggering point , currently I'm working in the release branch so when i'll merge these changes in to master then the master branch only need to create a bamboo build otherwise no build will generate in both branch.

please help how to make this in bamboo spec nt using UI 

Trigger bamboo build only when pull request is merged with master