This feature is missing, it will be added in next bug-fix version:
https://ugene.unipro.ru/tracker/browse/UGENE-1156For now it's possible to do the following trick:
1) From context menu in Sequence View select
Cloning->Create Fragment. In appeared dialog leave default settings and click OK. This will result in creating a DNA fragment annotation.
2) From context menu in Sequence View select
Cloning->Construct molecule. In appeared dialog add the created fragment to molecule fragment list and set option
Make circular selected. Click OK. This will result in creating a circular sequence, which equals to the source.