After the helicopter sequence you will fall onto a moving truck. In this next section there are various vehicles such as military and civilian. You can’t hijack the same civilian vehicle that has a different colour scheme, it must be a different vehicle entirely. Whilst driving on route be sure to lookout on the right side of the road as there can be civilian vehicles parked waiting to be commandeered.
Best to do this on Recruit difficulty.
Below are images of what the vehicles you can hijack look like:
Vehicle 1
Vehicle 2
Vehicle 3
Vehicle 4
Vehicle 5
Vehicle 6
If you are still having trouble, feel free to use this video as a reference.
Hope this helped, thanks again.