Back then using v1, users were able to trigger build for PR using PR number. - For example,
curl -X POST --header "Content-Type: application/json" -u $CIRCLE_TOKEN -d '{ "branch" : "pull/2"}' https://circleci.com/api/v2/project/<vcs>/<username>/<project>/pipeline
It's not anymore available for v2 and this function should be supported for PR builds.
CCI-I-1280