Welcome in the forum, Julian.
To maintain 0.25% accuracy required for USBFS you will need to trim the IMO using the optoin "to USB"
use an external crystal, either a 24MHz or a 32kHz watch crystal.
i try trim usb get clock stop for period of max divisor, 64k, into 48000000.
so how does trim work?
Just select "USB" for the "Trim with:" list box of the "IMO" in the "Configure System Clocks" dialogue.
Please don't forget to enable the ILO too.
The PSoC 4L periodically compares the IMO period difference and the SOF packet timing and trims IMO. The ILO makes the timing to compare and trim.