You need to download the Logitech GHUB. After you installed it open it and you should see your Mouse. Then Click on your Mouse and the you should see a small Window on the left with the Assignments and on the right next to it your Mouse with all the function like Primary click for your Left Mouse Button. Then Click on view 2 which is under your Mouse. Then You see your Mouse from the left side. Click on the white point from where the white line goes to Forward. Forward should now stand there in Blue. Under it you see now Use Default and Disable. Click on Disable and Forward with the white Line is gone. Then click in the Assignments Window on Keys and click on the Searchbar and write F13. Then there should be only one entry seen which says F13. Go with your Cursor over the F13 and make a leftclick and hold it. Then move your Cursor while you hold it over to the white circle which is empty. The white circle should get a bit bigger and the white should be now blue. Now let the left mouse button go and it should assign the F13 key to your G5 button. Now you only need to write F13 in your Config and it should work.