You can get them on the same plane by selecting them and then run Transform >Align and pick in the Front view.

Then if you want to edit the curve to be tangent to the circle, create a tangent line from the circle:


The first 2 control points of the curve must be on that line for it to have the same tangent as the circle at that point:


Also if you cut the circle so it is an arc that ends at that spot you can use Construct > Blend like this:





- Michael