Ehm... I found the solution for this problem by myself.
I write it here so that anyone who comes after me can find it;)
To fill a form from the URL, it is necessary to pass in the link the names of the inputs and their respective values.
You can find the names of the inputs by searching the source code, or by inspecting the page with a browser.
- wpFromName is for the name field
- wpFromAddress is for the email field
- wpSubject is for the subject field
- wpText is for the subject field
Example:
https://mediawiki.org/wiki/Special:Contact?wpFromName=James&wpFromAddress=info@example.com&wpSubject=Test&wpText=this_is_a_test
This will fill your form with:
- James as name
- info@example.com as email
- Test as subject
- this_is_a_test as message text