Are you wondering if you should allow Windows Defender to run along side of, with, F-Secure for better protection?
It should be turned off, as to not cause system instability, or slow downs due to 2 scanners running at the same time, even though Windows Defender is purely a spyware scanner. To use the free on demand version of Malwarebytes would be a good option. Or, to use an online browser based scanner for a second opinion, is another option. Be sure to disable F-Secure's real time scanner (Computer Security/Settings/Virus and Spyware Scanning) during that time, as to not have both of them scanning the same accessed files.
As baroque-quest mentioned, F-Secure should have disabled that upon install, but please check to confirm it's real time protection is turned off.
F-Secure will turn off Defender. No need to do it yourself.
You're welcome, Bengt. Thank you for posting back, and for the kind words. I'm glad you were helped