Zeno,
You can assign the temperature difference as a temperature differential or a power (time-constant or time-varying). It is not possible, however, to input a function. In older FEFLOW versions the user interface only allowed for a fixed/variable inlet temperature. For these, there is a free plug-in that provides the possibility to apply a temperature differential. To apply a specific function, you'd have to develop your own plug-in.
Ciao,
Peter