Solved
Unfortunately, this obvious feature is not available in Prime.
One of the many things that was readily available in real Mathcad many, many years ago but did not made its way into Prime yet.
As a workaround you may position a textfield in front of the area.
In a thread about a week ago Terry suggested to use a picture of a line to be placed in front of the area: https://community.ptc.com/t5/Mathcad/Hidden-Area-formatting/m-p/891925/highlight/true#M208020
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