We offer an excellent feature to do cross domain tracking, learn more in this faq
This feature works automatically in most cases, by updating the links in the DOM and adding the parameter
&pk_vid=xyz to all links going to the other known domains.
To make it possible to use cross domain tracking even in a complex app/website that has dynamic links, then
&pk_vid=xyzwhich they can then manually append to their cross-domain links.