Hi @_Ferrari_,
You can setup an external oscillator for the MCXA153 by going into ConfigTools and Clocks. Then on the diagram window, simply right click on the SOSC and click "Enable SCG.SOSC". A window will open where you can set up the frequency of your external clock:

Make sure that on the Clocks Table window you set the External Reference Mode to Crystal oscillator:

Then in Pins Tool, simply add the XTAL and EXTAL pins as the SCG0 function and you'll be set:

BR,
Edwin.