Hello Florian,
It is not possible to define the prescale factor for the STM32 HRTIM unit using the block from the target library. As you surmised correctly, we automatically choose the prescale factor such that we can achieve the highest resolution for the desired frequency, with some margin to allow for frequency variation.
The only way to manually specify the prescale factor while using the STM32 TSP is to generate code into an STM32 Cube IDE project and subsequently modify the code before programming the MCU from there. To find the corresponding lines of code, search for the functions LL_HRTIM_TIM_SetPrescaler() and LL_HRTIM_TIM_SetPeriod().