AN2606 describes this process for the STM32L4R in section 65. The bootloader can be activated using pattern 6 (section 4.1), which describes the possibilities for activation.
Good luck!
Regards
/Peter
<edit>As I've just seen, you've essentially asked the same question.</edit>
In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.