Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32U599/TIM15/TISEL#0x0
input selection register
selects tim_ti1_in[0…15] input
selects tim_ti2_in[0…15] input
https://github.com/cmsis-svd/cmsis-svd-data