Also i think the terms existing/new url is a little confusing. You could replace "new url" with "old url" in a future version. Because usually i think "new url" as the new url that will replace the old url that doesn;t exist anymore.
50% of our users agree with you on what old and new is. 50% of our users disagree with you. So, I named them existing and new. Existing is unambiguous: it either EXISTS before you create a redirection or it doesn't. It cannot exist if it's invalid and throws a 404.
Regarding your problem, I don't know which is the URL you are using now. I will need the URL you are using now to really help you. I know what you tried but I'm not sure if you are doing something the other way around than what you should be doing.
Let's say the page is currently accessible
without a redirection as http://www.example.com/apple. Let's say that you actually want someone typing http://www.example.com/orange to see the same page. You need to create a redirection where
Existing URL: apple (WITHOUT the domain name, WITHOUT the leading slash)
New URL: orange (WITHOUT the domain name, WITHOUT the leading slash)
What it really does. When someone visits http://www.example.com/orange they are redirected to http://www.example.com/apple. The http://www.example.com/apple page is displayed on their browser and the URL http://www.example.com/apple is shown on their browser's address bar.
So, knowing that, the second thing you tried will never work because the new URL is invalid. The first thing you tried would have this effect: if the user visits http://www.example.com/πυροσβεστήρες-ξηράς-σκόνης they will be redirected to http://www.example.com/πυροσβεστήρες/ξηράς-σκόνης
Also keep in mind that this will only work if the Admin Tools plugin is published before your SEF plugin. If, for example, you are using sh404SEF and its plugin is before Admin Tools then the redirection feature will not work because sh404SEF will have handled the URL before we have the chance to see it.
Nicholas K. Dionysopoulos
Lead Developer and Director
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