open Task Scheduler (it comes included with windows), and on the right side, click Create Task.

Name and description dont matter, call it whatever you like.
go to Triggers, click New. Set it to "Begin the task: On idle". You dont need to check any of the other boxes, just make sure "Enabled" is on.

Before you move to this next step, you need to find the location of the .exe of your browser of choice, i recommend using one that you arent already using as your main browser to make sure this doesnt interfere with anything. Im using Chrome for this, so my path is C:/Program Files/Google/Chrome/Application/chrome.exe. I initially tried Firefox but it didnt like running via command for some reason.

next, to Actions, hit New, set Action to "Start a program."

(for this next step im using chrome, but this should work with most any browser that has a kiosk mode, it might just need different arguments)

under "Program/script", write this:
C:/Program Files/Google/Chrome/Application/chrome.exe -kiosk helldiverscompanion.com
this launches the program (chrome) in kiosk mode (fullscreen with no borders or other browser buttons) and tells it to launch directly to a website (helldiverscompanion, in this case)

next, go to Conditions. check "Start the task only if the computer is idle for:" then set the time to whatever you like. Any other checkbox there is optional and up to your preference. Then just hit OK and save it all.

Be sure to test it by selecting it in the list on the main screen of Task Scheduler, then hitting "Run" on the right side menu. if it works here, it should work when your timer runs out.

it may fail to launch in true fullscreen kiosk mode, but this is only because Task Scheduler is set to stay on top. From my experience, if you have anything else focused when this task goes off, it will still open above everything else just like a screensaver - the Task Scheduler window seems to be the only exception.

if everything was correct, the result should be that when your specified idle time is met, the browser of your choice will open a borderless, fullscreen window of helldiverscompanion.

as this is not a "real" screensaver, it may or may not close upon action - there is a setting for this, but ive found it to be unreliable. Worst case scenario, youll just have to alt-f4 to close it.

If you have any questions, comment below or add me! Id love to help.