How can I extrude a circle into a rectangle but i want it to be like half a sphere inside

Simplest way I can think of doing this is to extrude the rectangle and then Revolve-remove a revolution of a half-circle:

https://cad.onshape.com/documents/6a799cc20592ea76812848f4/w/0b0d21da9f86e5b48079db80/e/9536ec8b80f9f467bc61dca7

You can also create the sphere as a 'New' in the revolve and Boolean-remove it later (as shown in the second part studio).

Instructions for how to explore this example:

1. Copy the document
2. In your own copy of the document, use the rollback bar to go step-by-step along the construction of the example ( https://cad.onshape.com/help/Content/feature_list.htm )
3. Double click on any of the features in the feature list to enter 'edit mode' on them, which will give you access to see their input selections, and see the output of that feature visually.