Does a conference poster count as a publication for PhD application?
It's definitely a good idea to include such a poster in your CV or mention it elsewhere in your application (e.g. discuss it and the work in the personal statement). However, I would refrain from listing anything under "publications" unless it has been peer reviewed and published in a reputable journal.

I think in this particular case, the application is asking for references to papers rather than posters, but it may be worthwhile emailing the admissions people at the department to ask where you should include the poster.