Extrude curved surface and snapping to model in sketch mode

Hello,
i am new to this program but i pretty know lot functions from C4D and few years ago Sketchup, so i gues i should not be so stupid but its 2 days and i still cant find good solution.

1) Extrude curved surface

I need to make two small support by extruding two lines on both side. So i was trying to make sketch which will copy size of line i need to extrude and "somehow" extrude this part from model

(i tryed this but this is not solution https://www.youtube.com/watch?v=byjrPlJKP8A)

I would like to be able to use this kind of pulling https://youtu.be/aBj3BF2Khf0?t=296

So in result it should be able to copy that surface like in this example:

2) Snaping to geometrics

While i was trying to make sketch, i was unable to snap rectangle to bottom point of model. I am also unable to find working solution.

Thanks in advance !

My project is here: https://cad.onshape.com/documents/791923bbfb67d619abdecfc0/w/3ff26eb271a4fd085c0a06e1/e/72be24b71c76d375672b81e5?renderMode=0&uiState=6866d028abc77a5fb9d603c1