What is the version of PSoC Creator you are using?
The UART component has a Hardware FIFO of length 4. If the Rx Buffer size required is to be more than 4, then internal Rx Interrupt is used to store the received buffer into RAM which forms the Rx Buffer.
If you increase the Rx Buffer Size to more than 4, then the Interrupt "RX-On Byte Received" will be enabled by default.
The operation which you want to conduct upon every Rx Receive Interrupt can be stated in the isr_1 interrupt service routine.