First: Look at the schematic for the kit to determine which MCU pins are connected to the push button. Set these pins as inputs in the configurator. If you are using the BSP, these will probably already be configured correctly.
Next: In the configurator make sure they are enabled for IRQ. Now define a name for the callback function associated with the interrupt. Create a callback routine that responds to the push button interrupt. You may want to use a de-bouncing routine to eliminate follow-on interrupts caused by a switch bounce.
For an example: The tone generator project includes a good example and description of a push button (section 3.1). A PDF description of the Application Example is available here: https://www.renesas.com/search/keyword-search.html#q=r12an0051
The project is located here: https://www.renesas.com/software/D6000226.html