The two ways I do parallax backgrounds with Fusion are using layer settings, ooor Background System Box objects.
Way 1: Layers:
Make a new layer, use any objects you want to make the graphics (Active, Backdrop, etc), then go to layer settings and edit X and Y coefficient to edit how fast the layer scrolls. Something like 0.90 can give a nice slow scroll.
Pros = Quick and easy. no events needed
Cons = You need a new layer for every differently scrolling level of parallax
Way 2: Background system box + events:
Make a Background System Box, change Fill and Border colors to None, set Image to Pattern, hit Edit and make or import or copypaste your graphics in. Then resize the object biiiig.
Next go to events, find or make an Always event after any camera related events, Background System Box > Set Position, and set X to X Left Frame * 0.90, and set Y to Y Top Frame * 0.90. Finetune the 0.90 values to whatever works the best for you in-game.
Pros = Only a single layer needed. Allows for a bit more control over some things.
Cons = Animating stuff is troublesome with this method
That 0.90 value I gave is really just an example, try values between like 0.50 and 0.99 to see what works for what you're going for. If you want something to stay completely still and not scroll at all set the value to 0.
There's ofcourse many ways to improve either of these methods to add scrolling for a water or clouds etc, but yeah hey.