I tried your image and I was able to pick a rectangle on the left border which I assume only contains orange mask. Your picture is a bit tricky since it has different areas on the border which I don’t know what they are, but since you own the negative you should know which part is really the orange mask.
What I found difficult as well is that I had to tweak the exposure a lot for every of the areas. I tried all of them, but none seemed to be a good reference.
In the picture you can see (it’s a bit difficult to observe) a rectangular frame on the left side, where I used the colour picker of the invert module to select an area. Just hold your mouse button while you are “painting” the rectangle. The white balance is a bit off, but in principle it works.
Please, don’t do that, you will loose a lot of flexibility and VueScan’s scratch removal algorithm is, let’s call it, basic. I did a lot of tests and was able to get so much better results with G’mic (see this thread: Scanned image scratch removal with “ICE”, especially the later results).