How to add the getproperties attribute in the mashup?

Yan He, are you on 7.2.0 by any chance? We had a known issue where services for a Thing with space(s) in its name does not render correctly on the Mashup builder, and has been corrected since.

Can you try upgrading to our newer 7.2.x release and retry? If you can't upgrade at the moment, you would need to use a Thing without spaces in its name as a workaround.