How to run build/test with source branch of pull request?

Maybe I’m missing something very obvious here, but I can’t seem to find a way to build source branch when a PR is created.

I’m trying to test the source branch of PR before merging it into master.

PS: using exec pipeline

If you do not desire the default clone behavior, you can disable the clone step and add a step with your own clone commands as shown here:
https://docs.drone.io/pipeline/exec/syntax/cloning/#custom-logic