Welcome, Tim.
I don’t believe that Selenium tone is available using Silver Efex Pro(?) - - but, it is available with DxO’s Film Pack … and readily applied as a component of split-toning … as shown below;
…
John M
Edit: Correction - I see here that a Selenium tone is available in Silver Efex Pro … All the same, tho, it looks to be more completely implemented with Film Pack. I suggest trialling both tools.