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).
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