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Please see the official X‑Plane Support page for help.
Custom markings is a bit more advanced than I've ever gotten with WED. If you haven't checked it out, you might find Jan Vogel's WED tutorials helpful--this one on polygons might be related to your issue. You may also find more advanced help in the X-Plane.org scenery forum.
I believe you will need a custom draped polygon and then need to use the texture mapping box to place parts of it where you want it. Here is the only information on this in the manual:
The Texture tab is for texture mapping a custom draped polygon file. Select your file in the library pane, then select Use Texture Map in the tool defaults pane. After drawing your shape with the polygon tool, use the bounding box in the texture tab to select what part of the original image is displayed in the shape. Use the Snap to Vertices options for the most precision. Note that this tab only works with custom draped polygons that have been drawn with the Texture Map box checked—orthophotos and standard Laminar Research .pol files are not applicable. Custom scenery that uses these texture mapped files are not eligible to upload to the Airport Scenery Gateway.
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