Please follow official Steam guide in order to disable DVR in Windows 10.
Unfortunately none of methods worked permanently for me (together with uninstalling xbox app in windows store), I had to edit registry every restart which was annoying.
If for whatever reason it also doesn't work for you, or DVR is being re-enabled on every Windows startup you can use this super simple app.
Application comes with handy installer, DOWNLOAD HERE
If you want to compile it by yourself remember to add task to Task Scheduler (run with highest priveledges)
Why don't you just use .reg file?
I did for long time. Then I added .reg file to startup and it asked me for admin permissions every time. Then I created few lines Lazaurs application and added scheduled task manually. And today I created app with handy installer which is doing everything for you, hassle free.