This exists now. In case anybody finds the above instructions confusing, here is the step-by-step on how to set up "click and add" price alerts.
Steps:
1) Right click on a chart > Chart Area > Config Area. Check the box in the bottom-right that says "Enable placing alert on chart".
2) Next, you must create a hotkey (Setup > Hot Key > Add New Item). I have simple scripts for when price crosses above or below my alert price. I'll share them:
Price crosses ABOVE alert price: AlertName=newalert;AlertType=LastPrice;AlertOperator=>=;AddAlert
Price crosses BELOW alert price: AlertName=newalert;AlertType=LastPrice;AlertOperator=<=;AddAlert
3) Open the Alerts window (Tools > Alert & Trigger). This window NEEDS to be open for the hotkeys to work, so I now just have it permanently fixed in my Desktop layout.
4) Now, when you press your hotkey, an arrow with an "A" next to it will show up on the chart, and you simply click the price where you'd like the alert to be placed. (NOTE: The first time you click, you'll get an error message that says "Placing alert failed". Just click again and it will work. This is a bug and I've reported it to DAS already.