In this thread, I will show you the ropes

What you're looking for (assuming you're tying the bike to at least two pole-like objects at the back of your truck) is called lashing. Essentially, you tie a clove hitch knot, wrap the rope around both poles in a cycle, and then tie it off.

This troop put together images, as well as an animated clove hitch, to show you how to do that - https://www.bsatroop780.org/skills/Lashing.html