I think there is already a reasonable answer, but I wanted to add my own experience in the medical field. I don't think this is quite as straightforward as @Buffy makes it out to be.
It is not standard practice, even for standalone abstracts, to have citations in medicine. It isn't entirely uncommon though... Depending on where you are submitting your abstract, the rules vary. Some conferences will explicitly say you should not have citations in the abstract - no matter what. Some will say you can. Most (smaller) conferences do not mention it but the general expectation is that there will be no citations. This expectation exists with the understanding that there will be uncited background info in the abstract. Whatever you opinions are about this, it is what it is.
Since your conference doesn't specify, I think you should try to include a citation. It is best practice and it increases visibility of the other paper. You can do this either with a normal, full citation at the end, or with some sort of abbreviated in-text citation. Whether or not they accept this depends on how strict the rules are (this usually directly correlates with how large/well-respected/well-run of a conference it is). I have never had an issue including citations if they are not explicitly prohibited though.
If they don't like it (which would be surprising), include the citation on the poster. In that case, I would also consider rewriting your abstract to not rely so heavily on your prior work. You can still mention it, but save the details for the poster and briefly state "we did x, y, z prior work in previously published paper A" and move on.