How to get a soft light leaks like in this video

I don't see anything in this frame that could not be recreated with a couple of blurred solids and Lens Flare if you manipulate the blend modes and opacity. Here's a 3-minute workup using two copies of animated lens flare on top of a shape layer with a semi-transparent gradient full, Gaussian Blur, Turbulent Displace, and Hue and Saturation on top of a composite made from a black and white image of a singer on top of an Ellipse on top of a starfield. The only thing missing is the Audio Track and Audio Waveform. I don't believe any of the overlays in the video sample are stock light leaks footage.

Not saying that using stock footage isn't a good idea, just saying that it is not necessary for that kind of a shot.

Here's a tutorial about creating lights leaks using nothing but Premiere: https://bit.ly/MakeLightLeaks