How can I allow users to add notes/comments when placing order?

[Imacloud]

Hi everyone, I’m trying to add a text input filed in the website, to allow users add in ordering note/comment when placing orders, so they can state their special requirements.

Something as shown in the image below, but for items without variants (i.e. for placing Sales Order but not quotation) :

Any idea how I can achieve that?

Thank you!