One of the options listed under IPTC Contact panel allows you to insert a link to a website. Normally, it would be your website, but there's no reason why it can't point to any website or page on a website.
Thank you for your answer.
To be more specific: I do not want to link to a website. I would like to link to a text file which includes the picture's deposited right of utilisation. The text file is on my PC. But the deposited informations are too big to write them down under IPTC. Hope there might be a way as easy as to a website.
To build on Ian's suggestion, you can include URLs to files on the computer, e.g.
An easy way to construct such a URL is to go to your browser, give the Open File command (in Chrome, it's File > Open File), and then browse to the desired file. The browser will open the file, and you can copy the URL from its address bar.
If you don't like using the IPTC:Website field for this purpose, or if you want more than one field in which to store URLs, I think the Custom Metadata plugin would provide that capability. The author is very helpful and a frequent contributor here.
Finally, you might consider my Any File plugin. It lets you directly import files of any type into your catalog. For example, you could import the release PDFs for an image and stack the release and the image together.