It cannot be set by default - the blueprint is a template, it does not know what’s in the level. Only once you instantiate it, one can assign values to Instance Editable references.
But you can select multiple actors in the Outliner and assign the same thing in one go. Can be cumbersome, though.
If there’s only one then I wouldn’t even bother setting anything else up and:
And now all Decoy actors have a refence to The One mutant there is.
AND if there’s only one, you do not need an Interface as @bumbumgoesnuts suggests. You already have a reference of the one correct type.
But, of course, you may have other plans for later.