You may use the field «Remaining URL» to add URL-parameters like ?eventcategory=4.

I would also recommend the use of Remaining URL field. That will save your efforts.

In addition to suggetion by @Thomas you can also use it like ?category=yourcategoryname1&category=yourcategoryname2 for same category or for multiple categories ?category1=yourcategoryname1&category1=yourcategoryname2&category2=yourcategoryname1&category2=yourcategoryname2

Thanks Tomas and Praful,

i have verified that this approach works, i like it. Just i'm still waiting for the ok of the customer. Maybe he finds this approach too complicated for the cms editors, they need to know that the Event-Category information must be appended as Url Parameter. So i've hoped for an easier way to customize the link-editor. I guess you can do it with a dojo widget, can't you? I'm just not very familiar with it and have hoped that someone could show this approach. 

Another way would be to add the property to a custom Block we already have(Link-Block). Then i needed to support drag&drop on the TinyMCE editor, so implement javascript logic to add the custom data-attribute when the editor drops the block. This must be done also at serverside when the link is not dropped onto the editor but added to a content area. This is very complicated and prone to errors(regex and HTML can be a nightmare).

So i see that your way with using the "Remaining Url" is pretty straightforward and also easy to extend.