From where I can get the link of online application?

According to me, for mechanical Stream there are various psu's which starts their application process as soon as Gate Admit card is released. Because for registration you have to provide your registration number.I am providing you the list of various psu's and then you can check them out.BARC:- https://www.i-register.co.in/BARCREGENG19/frmdeclare.aspxBEML Limited or Bharat Earth Movers LimitedBHEL - 12th FEb 2018 BNPM- Not announcedBPCL- Not announcedDDA(Delhi Development authority) - 1st Jan 2019DRDO EIL( Engineers India Limited)GSECL (Gujrat State Electricity Corporation Limited)HPCL HALIOCL( Form filling date commences- 4th jan 2019 and ends on 11th feb 2019)IRCONMDL( Form filling date commences- 9th jan and ends on 7th feb)NALCONFL NTPC( Form filling commences- 10th jan and ends on 10th feb)There is no websites for all the PSU's, you have to check them all in their respective websites.