I have only the trial version of the application and I would like to know how I can find a license key.
Hm... are you sure you are talking about FDM? As its name says, Free Download Manager is completely free, you do not need to buy it.
You are correct Joan TG. Free Download Manager is FREE and the license is called a FREE LICENSE. I use it and find very useful with our slow connection - to download in multiple sections to increase speed and for its ability to resume downloads even when the connection is lost for a while.
Its A free software. I think you are talking about some other software
As Joan 9 says, the application is free of charge and you will not need a license key in order to use it. I suggest you remove the application you have from your computer (Control Panel > Uninstall a program) and download the latest version from an official source.