Are you just asking how you to implement the apply function from above which should call a function qualified by its fully qualified class and method name given some arguments?
Given your immediate question, cbley is right that reflection is the immediate and simplest implementation answer. However, there are many problem space contexts within which depending upon reflection is actually either a code smell or even an anti-pattern. For example, if you intend to deploy your results into many production server contexts (especially with high-security requirements) or outside the JVM (Scala.js, Scala Native, Spark, etc.), you will find reflection significantly impeding, if not outright blocking, your movement forward.
And finally, another architecture and/or design option is to deeply understand the problem space and exploit the many FP (Functional Programming) alternatives which offer almost all the benefits of reflection with none of the code smell nor anti-patterns.
Agreed. Are you coming from an Erlang background, by any chance? (The question sounds Erlang-ish.) If so, the Scala language is very, very different, but the Akka library is a descendant of Erlang, and will look a bit more familiar.