From the web site. I'd never heard of the magazine before today, but the following is fairly standard for this kind of publication.

SUBMISSIONS
Jacobin welcomes unsolicited essays directed to our submissions inbox. Completed manuscripts are welcomed, most of our articles are under 2,000 words. Send a short abstract introducing the piece in your email. The articles cannot have footnotes. We try to respond as quickly as possible, but receive a large volume of submissions. We do not consider simultaneous submissions. If your draft needs to be considered or run within a week include “[time-sensitive]” in the subject heading.

Contact us at submissions [at] jacobin.com