Basically, with a function map, you would create the map at the start of the layout by using:
CONDITION
System ▶︎ On Start of Layout
ACTION
Functions ▶︎ Map “FunctionMap” string “FunctionOne” to FunctionOne
This creates a map called FunctionMap, and then maps the function called FunctionOne to the string "FunctionOne". Then, when you want to call the function using the string you'd use the Call Mapped Function action, specifying the map you need and the string. If you wanted to choose randomly from a set of strings, you can use the choose expression in the String parameter.
This example is adapted from the official Function Maps example.