I added “tk.de” but still the password was not filled automatically
If tk.de is literally what you added to the URL field, it is not a valid URL so that might explain this initial problem.
This might indicate a poor match with the information you have stored in the entry. For example, if there are 20 form fields stored in the entry but only a username and password field on the page, Kee refuses to auto-fill since it’s highly likely that the entry should not be filled in that form. Same applies if you have just a username and password but the form on the page has many fields. Take a look at the KeePass entry > Kee tab > Form fields tab.
Are there any means to lower this security?
Check out the troubleshooting guide for info regarding undesired password prompts.