I think specifically we need to know what processor you are trying to use as this differs from device to device.
The simplest and most generic answer would be to add the UART to your project and click on the checkbox to enable interrupts for the driver. After generating code you will have to set the callback which you want called when the interrupt occurs. After this you need to make sure you are enabling interrupts in your main code and it should work.
If you supply us with the details above I will post some screenshots for you on how to do this.
Just to show you the idea I picked the 16F18875 and added the EUSART as follows:
You can see I clicked next to "Enable EUSART Interrupts"
Then in my main I ensured the interrupts are enabled.
When I now run the code the ISR created by MCC is executed every time a byte is received. The ISR function is called EUSART_Receive_ISR and it is located in the eusart.c file. You can edit this function or replace it by setting a different function as ISR by calling EUSART_SetRxInterruptHandler if you want to change the behavior.