Another super fast way to do it, as shown in the demo below:
(Edit: Sorry I showed Lightroom Classic, I later realized this is the Camera Raw forum. But they have the same controls anyway, just translate them.)
1. Set up a Brush mask with the appropriate Size, Feather, etc. values for your light beam.
2. To make a perfectly straight beam of light, click (don’t drag!) the Brush where the beam starts, and then Shift-click where the beam ends. Painting a straight line by Shift-clicking the Brush tool is an old Photoshop trick that also works in Lightroom Classic and Camera Raw.
3. To remove the beam of light from the subject, click Subtract and then choose Select Subject. (If the subject isn’t correctly masked by Select Subject, do it more precisely using Select Objects or Select People.)

(Sorry about the image quality. The GIF animation I uploaded looks good, but the Adobe forum software degrades the colors for some reason. It didn’t need to be recompressed, what I uploaded is only 1.2MB.)