Currently, it is not possible to define a property for attribute type Lookup.
Please add support for this type
Currently planned for the next semester (April 2020) - please note that actual delivery dates can change .
Lookup has complex set of APIs to function in full capacity and we are evaluating if we can release the support in staged manner. This workitem is under review currently.
I do understand that some type of attributes can be harder to integrate (especially regarding Canvas app), but I really think all data type from CDS should be supported