Define construction line and not interfere profile detection

Hey, I want to model a symmetric slider with cambered top surface. So I draw a center line and ensure the rectange and the circle to be constrained.

I used to create construction line in other 3D MCAD, and it will not disrupt downstream operation, like extrude the profile.
I couldn't find the way to define a line as construction geometry, here I create a center line indeed.
Refere to below image, it's very annoying when I tried to pick the profile to extrude. Any suggestions?

Thanks.