how can I use the external monitor when I turn the mac's lid off (the Mac is already connected the power adapter)

[Nomedella]

HELP！ I've bought the software, but I always turn the mac's lid off, and use an external monitor which connect my Mac with HDMI. In that case, once I turn off the mac's lid and then the external monitor will turn off too, is this because I setting something wrong?(I have turn ALL the switch on in the "charge" icon)