How do I fix points on curves and cursor size

[Redshift]

Hello,

When I have the Curves Transformation box open and I run the cursor over an image, how can I fix that point on the Curves? I've tried various combinations of button clicks but can't find how to do it.

Also, for us people with wonky eyes i.e. me, is it possible to make the cursor cross hairs larger? I spend most of my time looking for the cursor 
I've changed the colour to tomato red which helps a bit but ideally I would like to make it larger.

Thank you
Dave

[harist]

Tried this recently (clicking on an image with the curves process open) and it just shows on the curves window the corresponding point. Couldn't find a way to fix this point either. Once you "declick" the point is gone.

Tasos