Find your paper on PubMed. The PubMed ID is an 8 digit number that appears in the URL at the end, ncbi.nlm.nih.gov/pubmed/RIGHT_HERE
. It also appears just below the abstract, "PMID: 8-digit number." You are asked to provide the PubMed ID for your paper. A URL that takes me to that specific paper on any web site would be OK (typically, this would be a bibliographic site such as CiteULike or the journal).