Hello,
you should have a screw that is 45mm (thickness rule) without thread, pre-drill the rule, most wood screws can manage to cut into the steel rule, but it will sit moderately, you can get a maximum of 2 screws on your rule piece if it is set with 45cc,
a bead of construction adhesive solves this, alternatively, find out how many layers of drywall there are in the wall, buy molly plugs for 1 or 2 layers of drywall, pre-drill the rule with the same diameter as the screw for the molly plug,
then countersink the screw head in the rule with a suitable drill so that the screw reaches into the plug, you can also complement with a bead of construction adhesive for a "bulletproof" installation.
Good Luck m8y
Regards
IC