Modify ticket links via REST API

Hi. I have used the REST API to successfully create a RefersTo link to an existing ticket but it’s only possible for users with special rights. My question is:

What right does the user need to update ticket links?

I receive the following error: “RefersTo: Permission Denied” for some users.

I know it is possible if I set ModifyTicket right for the group the user belongs to, but I want to know if there is any finer right that is only related to creating links.

BTW, I’m using REST 1.0