Unfortunately, this is not supported in the ASP.NET version of the Scheduler. In the JavaScript version, it's possible to customize the behavior using onGridMouseDown:
https://api.daypilot.org/daypilot-scheduler-ongridmousedown/
The implementation could look like this:
dp.onGridMouseDown = function(args) {
if (args.button === "left") {
args.action = "None";
}
if (args.button === "right") {
args.action = "TimeRangeSelect";
}
};