Better way to create signature for preview than holding a paper in front of the camera
It can't be done directly.

Best authority I can find to back up this statement is Linc Davis, on the Apple Support Communities. - https://discussions.apple.com/thread/3667416

It is possible to create one that way in Acrobat Pro/DC, but I'm not then certain if that would then also work in Preview. It would depend if Acrobat creates the signature as a true Keychain Signature item.

It's not clear from Acrobat Help / 
Sign PDF documents but implies Adobe use their own cloud service to keep tabs on signatures rather than the Keychain.