Citing your own unpublished work (poster) I've been asked to acknowledge my own unpublished work but am not sure how to do this.

In short I've received a review on one of my papers that states it received favorable reviews and should be suitable for publication. However, the editor notes that the title and first two sentences of my abstract are identical to those from a poster I presented at a conference last year as well as some sentences fragments throughout the paper. The poster represented the early stages of the final manuscript and is not at all representative of the final manuscript. They've asked me to acknowledge the conference abstract in the paper.  It feels a bit out of place to just put the poster in the references section since it was not actually published and I'm not sure of an unobtrusive way to mention in the paper that the early work for this project was presented at a conference. I've written a response letter and am leaning towards putting the poster in the references but would appreciate any advice.

The field I'm working in is statistics if that has any bearing on the best way to do this.
You can refer to it without putting it in the bibliography. A typical way to do so would be in a title-footnote that says something like "a previous version of this paper was circulated under the name X, presented at conference Y, etc."