For the simplest usage I imagine user would type only input in-game sensitivity and FOV (maybe dpi also if it matters?). There are many values that can be provided by the game itself and assume that same settings are used in the input game. Resolution, aspect ratio, even win pointer speed seems easy to get in C# (Unity): stackoverflow.com
I guess for now I will open Steam overlay with your website
@Edit
Would be good if api also converted fov. Imo this could be the case also on the website and this value converted: