You post your own ads by going to www.xing.com/xas, specifying your target groups, uploading images and adding text, and setting a budget limit. Ads will then appear, e.g. in your target group's network news feed or other areas on the XING website and in the XING apps.
You can choose either your personal profile or your company profile to be shown as the ad poster when creating an ad. But please bear in mind that you need to be an administrator or editor of the company profile to do so.