Hi Bloodlust, so just to give you more information on your particular shape, in NURBS modeling the way you'd make a dip in your object there would be to draw a 2D profile curve something like this:



Then use Construct > Revolve (turn off the "Cap ends" option in Revolve) to make a type of bulged disc shape from that, this will be a cutting object:



Now take your base extruded object, which you would want to have extruded some extended amount further than what you originally did, and position the disc where you want the top of your final object to be at. Then use Construct > Boolean > Difference to cut the extrusion with the disc:



You will then be left with the imprint of the disc in your extrusion as one combined object:



This is a common thing in NURBS modeling where you will want to construct an extended piece initially and then use a boolean to cut off some excess pieces to form the final shape. Usually you can do the boolean using a 2D curve as the cutting object but in this case you will need to build a revolved surface and use that surface as the cutting object because you don't want just a straight directional cut.

- Michael