Login to your Jimdo account

Open your site menu, then click "Settings".
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Select "Edit Head"

Copy the head tag script and paste it in the appropriate area. It should look like this:
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Save your changes.

Then, go to the page you want the button to appear on.

Click "Add element" in a section where you want the button to appear.

In the "More elements and Add-ons" section, select "Widget / HTML"

Copy the body tag script and the button code and paste them together here. It should look like this (booking widget v2):
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Save your changes, then test your button.