How to set dongle to boot mode?

Greetings! Sorry for the noob question here, but how can I enable boot mode for my nRF52840 Dongle? I tried pressing the large obvious button and nothing happened. I tried everything around it and nothing seems to be a button. It's embarrassing, but I just can't figure it out! I'm attaching the dongle pic and the pinout, if it helps. Thanks!

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