https://downloads.dell.com/manuals/all-products/esuprt_display_projector/esuprt_display/alienware-34-monitor-aw3418dw_user%27s-guide_en-us.pdf

Sorry I read that too fast.  I don't know if it's the same for yours  (don't have that one), but on my newest Dell monitor (U series, manufactured 12/2019) you turn it off, then push the two buttons on the bottom left, then the power, then release all three at the same time.  Then to get the service menu box, you push button #1.  

Looks like this: 

Yes that is the screen but the sequence tio reach it is different. Thanks anyway.

Marco