For JSTOR, look for the stable URL. It is found in the browser bar address, in the citation area, and the "Cite this item" link.
Example while on campus:
1. Locate the stable URL: https://www.jstor.org/stable/43284491
2. Add the library's proxy in front of the JSTOR stable URL:
http://libaccess.sjlibrary.org/login?url=https://www.jstor.org/stable/43284491
Example while off campus:
You may copy the URL in the browser bar -- it has the library proxy prefix in it.