How to trigger build ONLY on pull request but not on merge?


I am trying to run a build ONLY on pull requests to main, but the build triggers on BOTH pull requests and merge stages.
How do I reconfigure it to not run on merges to main? Thank you!

I have the following setup:
VCS roots:
    Default branch: refs/heads/main
    Branch specification: <empty>

VCS trigger:
    Trigger rules: +:folder1/folder2/**
    Branch filter: <empty>

Pull Request feature setup:
    Pull request filtering - By target branch: +:refs/heads/main

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