The second argument to FormFunction is function that expects a single Association (as is also true of APIFunction). The arguments coming to the FormFunction are available as named slot parameters, written like #Basis. It is not recommended to use uppercase letters to begin symbols, so I would also rename your FormFunction parameters to begin with a lowercase letter. Based on I would rewrite your code as follows:

simpleFunc[basis_, exponent_] := basis ^ exponent
foFuSiFu = FormFunction[{"basis" -> "Number", "exponent" -> "Number"}, simpleFunc[#basis, #exponent]&]

If your definition of simpleFunc and and your definition of foFuSiFu are in the same input, they need to be separated by a semi-colon. Better to put them in separate inputs.

In[1]:= simpleFunc[basis, exponent] := basis^exponent

In[2]:= foFuSiFu = FormFunction[{"Basis" -> "Number", "Exponent" -> "Number"}, simpleFunc]