Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32U031/TIM16/TIM16_TISEL#0x0
TI1SEL=B_0x0
TIM16 input selection register
selects TI1[0] to TI1[15] input
0 (B_0x0): TIM16_CH1 input
1 (B_0x1): LSI
2 (B_0x2): LSE
3 (B_0x3): RTC wakeup
4 (B_0x4): MCO2
https://github.com/cmsis-svd/cmsis-svd-data