How do script a key to press that makes my character switch from third to 1st person view?

For this to work properly, you're going to need two things UserInputService, and MaxZoomDistance, the first Service will allow us to Fire Events to Input related scenarios, and for the property, this allows us to restric their zooming ability, giving the impression of FirstPerson

local UserInputService = game:GetService("UserInputService")
local CamView = workspace.CurrentCamera.MaxZoomDistance
local CamView2 = workspace.CurrentCamera.MinZoomDistance
local ThirdP = 500
game.Players.PlayerAdded:Connect(function() --// As Player joins
   UserInputService.InputBegan:Connect(function(InputObject, GameProcessed)
      if not (GameProcessed) then --// if the Key was binded to any actions event; jumping typing then ignore
         if (InputObject.KeyCode == Enum.KeyCode.E) then --// KeyInput you desire
            CamView, CamView2= ThirdP
         end
      end
   end)
end)

Hope this helps! If so, don't forget to accept and upvote If there is anything confusing, please let me know!