I would use a strip of this tape behind the fender, installing it low enough that it's not seen when the two pieces are mated together. You may want to mount the tape to the molding first, leaving the red plastic shield on. Then set the molding in place and pull the tape as you go. Otherwise, it will be hard to set the molding in the correct position without the tape trying to stick when it's not adjusted to the correct position.
It looks like a custom painted fender, when done, you may or may not want to run a clear silicone bead in that joint for extra strength and keep the dirt out of the joint.
https://www.amazon.com/Double-HitLig...NsaWNrPXRydWU=