You can enter it as PMID: 123456 in the "Extra" field and citation styles that include PMID will pick it up. It will also move to its proper field in future versions of Zotero from there automatically (and this is how PMID is already imported from pubmed)
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PMID: 123456
in the "Extra" field and citation styles that include PMID will pick it up. It will also move to its proper field in future versions of Zotero from there automatically (and this is how PMID is already imported from pubmed)