In the field "Jobseeker criteria" on your XING profile page you can enter employers you'd like to work for. This helps recruiters on XING to match you with relevant job listings. Whether you are a Basic, Premium or ProJobs member, you can enter up to 50 employers of your choice.
The following link will lead you to the respective settings: https://www.xing.com/projobs/settings/jobwish/employer
By the way: jobseekers using XING will find more useful tips here.