Adds a custom property to the drawing file.
Specifies a unique name for the custom property. The name can contain up to 255 characters. The following characters are not permitted: asterisk (*), equal sign (=), less-than and greater-than signs (< >), slash (/), backslash (\), quotation marks ("), reverse quote (`), pipe sign (|), colon (:), and semicolon (;).
Specifies a value for the property. The value can be left blank.