Undergraduate resume for PhD application: Include publication in preparation?
I would definitely include the (future) publication in your applications. Add a section for publications to your CV. In this section, simply write


  [Title of your Paper], Author A, Author B and Author C, in prep.


This is a common shortening for "in preparation" that is used in academic CVs, talks and even published papers to refer to a work which is nearly ready for submission to a journal. You can talk about the work and your contribution to it in more detail in your personal statement.