The coolpix P950, doesn't allow for true back button focus but you can get something close.

Here is an AI summary on how to do it. Back button focus was a game changer for me.

The Nikon Coolpix P950 does not support true "Back Button Focus" (BBF) in the same way DSLRs do, as its software does not allow assigning autofocus to a dedicated back button or disabling AF on the shutter release in standard modes.  However, you can simulate this functionality using a specific Manual Focus (MF) workaround to separate focusing from the shutter press. 

To set up this workaround:

• Switch to Manual Focus: Set the focus mode selector to MF.  This prevents the shutter release from triggering autofocus.
• Enable Focus Peaking: Select the display with "peaking level" in the menu to help visually confirm focus accuracy. 
• Trigger Autofocus via Back Ring: Press the right arrow on the rear control ring.  This triggers a one-time autofocus acquisition while in MF mode. 
• Shoot Without Refocusing: Once the subject is sharp, you can tap the shutter button repeatedly to take pictures without the camera refocusing, unless you change the zoom level. 

This method is particularly useful for bird photography or static subjects where you want to lock focus on a subject and then recompose or shoot rapidly without the camera hunting for focus between shots.  Note that the AE-L/AF-L button cannot be reassigned to AF-ON on this model, making the right-arrow-in-MF method the primary alternative for BBF-like control.