STM32L475 One Pulse with timer

Hi,

i want to send only 1 Pulse and i thought to use a timer.

I use the Board B-L475 iot and i want to generate the pulse on TIM2_ch3 (PA2).

I had build the HAL with stm32cubemx.

Before the while(1) begin i have inserted the following Code:

HAL_StatusTypeDef ret=HAL_TIM_OC_Start(&htim2, TIM_CHANNEL_3);�?�?�?

but when i debug TIM2->are every time 0.

What had i do wrong???

Best Regards...

I have the following config: