I'm going to give you step by step instructions with the assumption that in your very FIRST post, you have 2 existing points along with the 'vertical' spline (as displayed in your post). I'm also assuming that the points & 'vertical' spline are all lying on the same plane & that you want to create a spline that ends at the same endpoint of the vertical spline.
Select the Spline icon.
From the Spline dialog, select Through Points.
From the Spline Through Points dialog under Curve Type, select Single Segment & then OK.
At the next dialog, you will prompted for the spline point creation method. Choose Point Constructor.
From the Point Constructor dialog, select the Point icon.
Now, select the left-most point followed by the right-most point. DO NOT HIT OK YET! With the Point Constructor dialog still showing, select the End Point icon & then select the end point of the 'vertical' spline then select OK.
The Specify Points dialog pops up & you want to select Yes & your spline should show up & pass through the 3 points you defined.
If this doesn't do the trick, then you haven't told us enough information or you need to be more descriptive of the existing geometry.
Tim Flater
Senior Designer
Enkei America, Inc.