It depends on the context, more specifically:
- What is the importance of such a poster in relation to your total list of published works? Do you believe that it was one of the top studies that you did ?
- Is there any in progress work that you are doing for expanding the work on this poster that you would like to cite in your application ?
- How relevant is this conference to the field you are applying ?
In my case, in my MsC application, I included my two conference posters in my CV as I did not have any relevant journal publication accepted at the time of the application. I would recommend using a different section for this, such as Peer-Reviewed Publications and Conference Presentations or Conference Proceedings. However, if you believe this is a minor work compared to what you have already produced I would not worry about that.
Lastly, I agree with @astronat that may be worthwhile contacting the department to ask what they are looking for in the application and how you could include this work to improve your CV.