Once you have submitted your marsh zone polygon, it will show in the popup. Or, if you have closed the popup, just click on the created polygon to select it (do not go into edit mode), then scroll down to the copy shape button.
Create new polygon, then Copy Shape:
Now, press the add data plus sign, choose the Planted marsh template, and it will automatically copy the shape to that layer, and open up the form for the newly created shape.
(Just don't add point as it will add another point at your location to the copied shape)
Basically, everything is the same as adding a new planted marsh polygon, but have copied the shape before selecting the add planted marsh template.
As you can see, there are two polygons here, but one is under the other so not visible.
R_