The RunParameter descriptor accepts a Project name. The autocompletion only works for Items from Jenkins root. The TextField has no context and therefore when jobs are organized in Folders, a full relative path from the Root (or an absolute path) needs to be provided. Even though the job is a sibling:

We could improve the auto completion to inject the context @AncestorInPath (similar to other auto completion for Project names like BuildTrigger and Parameterized Build Trigger).

https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/RunParameterDefinition.java#L154