Do you mean as a branch name filter? I am unfamiliar with how PRs are presented to Git inside the PR process - do they always get this as a prefix? If this is a branch name, then I wonder whether you would need an ignore rule as well - this is rather non-intuitive, but the docs do try to cover it.
See my demo here, particularly the only-deploy-tags section (and where that is injected into the workflows section).