Record the URL for the online serial in subfield $u of the 856 field.
First indicator = access method; "4" = HTTP
Second indicator = relationship; "0" indicates that the URL is for the item cataloged
Most commonly used subfields:
$u = uniform resource identifier (URI)
$z = public note
$3 = materials specified note
856 40 ‡u http://muse.jhu.edu/journals/poet/