Here is more detail.
https://codex.www.ads-software.com/Inserting_Images_into_Posts_and_Pages
An option could be to insert images that contain the shape that you want to use.
For example, an image containing the rectangle shape.
See Benachi’s (@utz119) replay above on how to do that.
Of if you are comfortable with html5, then you can use canvas tag to draw basic shapes, but that is a complicated way, and you need to have strong programming skills to do that.
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