No. Its on the spline mesh. Select the node where you create the spline mesh components, and enable it on that. Like this: If that still won't do it, I'm sorry but yeah this isn't working with spline meshes. In that case, I'm not sure how you should approach this, because this is the standard way otherwise.
Okay. Then sorry, but you can't sway a spline mesh, at least not in a straight forward way. You could use a manual bounding box in the material, so they would actually have the same weight, but this would be painful to work with, and I do not recommend it. The blueprint would need to set up the material parameters for this…