Manually configured External tool activities are no longer supported. Don't worry, this activity will still work as it is, but you can't make changes to the tool configuration here anymore.
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To make any changes to the tool, or to create new activities with it, the tool needs to be added to your course in Course > More > LTI External tools. Then, you will be able to create new activities, selecting the tool directly in the Activity chooser.
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You can read more about adding LTI External tools in the documentation <a href="{$a}" target="_blank">External tool</a>.