When you have your Camera selected, go to
Attribute Manager> Compositing> Crosshair (check)
Note: the options for color and size are available.
You could also just add a Null Object to your Camera, as below:
Eventually add a Protection Tag to the Null. (Null> Object> Point [size])
This Camera can be saved in the Object Manager>File> Save Object Preset…
If you like to have this request for the plugin as R20 anyway, please let me know, I will move the thread then to the CV-Toolbox Support.
All the best