Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32u5/STM32U535/TIM15/TISEL#0x0
input selection register
selects tim_ti1_in[0…15] input
selects tim_ti2_in[0…15] input
https://github.com/modm-io/cmsis-svd-stm32