1. Launch the Site Builder tool from your cPanel.
2. Go to the page where you want to insert your rectangular box.
3. Hover your mouse to 'Shape' element then drag and drop it to your page.
4. To edit its settings, click on the Pencil icon and choose Properties option.
5. In the Properties section or settings dialog box, you can change its border type, adjust its opacity, change its color and adjust the rounded corners.
6. Click on 'Apply' button to save the changes.