Sorry, my previous answer was for the iOS version of Textastic, but you seem to be asking for the Mac version of Textastic.
Use File -> Open… to open a folder and add it to the sidebar. Alternatively, you can drag and drop folders from Finder to Textastic's Dock icon to add it to the sidebar.
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Sorry, my previous answer was for the iOS version of Textastic, but you seem to be asking for the Mac version of Textastic.
Use File -> Open… to open a folder and add it to the sidebar. Alternatively, you can drag and drop folders from Finder to Textastic's Dock icon to add it to the sidebar.
Thank you. I should have specified the Mac version.