Search API for Lookup

I was able to retrieve the Company Description (Multiple lines of text) using the above url. I got 'CompanyDescriptionOWSMTXT' from search schema.

I also want to retrieve a lookup called sectors. How to get the search schema of that LookUp (sectors). This is my query:

url:"/sites/mysite/_api/search/query?querytext='contenttype:customtype+AND+ListId:4B5E162C-85CC-44DA-396FAE6662F0'
&rowlimit=10&startrow=1&TrimDuplicates=true&selectproperties='CompanyDescriptionOWSMTXT'" ;