How can I power on the laptop with the lid closed
 

I have an HP firefly 15 G7 t6o the displaylynk 4k

Hello,
Today this functionality is proprietary. It means you need an HP dock to wake up an HP laptop.
We expect things to be more universal in next generations.
I recommend you contact HP to ask if the HP USB-C/A Universal Dock G2 power button works with your HP laptop.
Kind regards,
Alban

Got this off Google : Open the Start menu and search for Control Panel. Navigate to Hardware and Sound > Power Options > Choose what closing the lid does. You can also type "Lid" into the Start menu to immediately find this menu. I assume you would have tried this already, is there a specific reason why you would like to this, maybe understanding why can provide another solution if the above doesn't work.