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Select Insert -> Text from the main menu or press this button

on the toolbar.
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A text element will be inserted to the page.
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To add text to it or edit the text in it, double-click on it to open it in the Text Element Editor. You can also right-click on it and select "Edit element" from the context menu to open it in the Text Element Editor.
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Enter or edit text and then press the "Save and Close" button to save the changes.