This is considered a vehicle side mission and is started by entering a vehicle rather than going to a phone. It is recommended you do these missions to build up your points though.
Enter the Tanker in north Law Island. You are told that the Tanker belongs to some white trash patriot group and is also armed with explosives. All you need to do it drop it off at the Pool Hall in central Kings.
S out, S big over bridges, W big, S small. END!
Video Strategy (External Link)
If the short directions were too hard to follow, here's the video: