If you can quickly finish your prayer just in time to join the Imam before he offers the first takbeer, you should continue you prayer. Otherwise, interrupt it.

When I enter masjid and peform Sunnah prayer like Tahiyyat al masjid, the Iqama for the Fard salah is made as I am still praying or in last rakah. Should I continue to finish my Sunnah prayer or should I end my Sunnah and join the congregation Fard salah?