It is possible to do. I have attached 2 screenshot, the first one shows the Starting state of the Railing, the second one shows the result. I show them in and Elevation Viewpoint because I can then show you exact distances and dimensions.
What you need to know that the Railing can consist of multiple Segments as defined by the Railing Reference Line, and the height of each segment is the same by default as set in the settings dialog. However, in Edit Mode you can display Segments and Nodes, then select individual segments and modify their Segment Height.
After you have placed a Railing that is associated to a Stair, you can add new nodes to the Railing. These new Nodes may or may not be associated to an element but the existing Nodes of the Railing will stay associated to their host Stair.
So, in an Elevation parallel with the Railing, select the Railing, click the lowest Node and use the "Continue Railing" Pet Palette command to move that node to achieve the Nodes shown on the second image. You need to create the new Node one Going distance to the left, so if your Going Depth is 350 mm, you have to create it 350 mm to the left. This is the point where the sloped Railing Reference Line Segment intersects the finish floor level.
Then use the command again to create another Node to the left again, creating a horizontal Railing Reference Line Segment; make it 500-600 mm to the left, we will adjust it later.
Now, go to 3D. We will need to do the next step in 3D because that is where Railing Segments and Nodes can be displayed. In 3D, select the Railing and enter Edit Mode. In the Edit Palette in the upper right, click "Segments and Nodes" to display them.
Click the leftmost segment (the horizontal Railing Reference Line segment) and in the Info Box or the Settings Dialog you can modify the Railing Segment Height to 900 mm. Then Exit Edit Mode.
Now, all is left is to adjust the position of the End Node of the Railing Reference Line so that the horizontal Top Railing length is exactly 300 mm. Go back to the Elevation View and on the second images you will see how I calculated where that End Node needs to be. You can Move this End Node with the Pet Palette command to its desired position.
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