Open the opportunity and click the Source URL button. That takes you to the original posting.
Some portals block direct links. MERX and Ontario Tenders are the main ones. For those, the button takes you to the portal itself rather than the specific opportunity, and you'll need to search for it there using the title.
Always confirm the closing date on the source portal before you bid. Portals issue amendments, extensions and cancellations directly, and the source posting is the authority on deadlines.