You can only define the functions’ interfaces (name, input/output parameters) in an interface. That’s their entire point.
You have to implement the interface in an existing Blueprint to define the actual function logic. To implement an interface, open the Blueprint’s Class Settings and Add the Interface you want. The available functions are then listed in the Interfaces section in the My Blueprint tab.
You can then define the function content as you like, but you won’t be able to rename the function or change the input/output parameters (because they’re defined by the interface).
Note that adding an interface will fail if your Blueprint already contains a function with the same name as one of the Interface functions.