It is currently not possible to set a normal event as completed.
However you can try the following workaround:
- In Week Calendar go to Settings > Auto color / Auto icon (which ever you prefer)
- Create a new rule "When Notes contains text Completed" use icon / color
- In Week Calendar go to Settings > Tools > Add menu item > Set notes
- Go back to the menu list and tap set notes
- Add "Completed" text in the box and give it a title.
- Select an event, then tap the Tools button at the bottom
- Select "Set notes" action and select "Completed" text. This should then trigger the auto icon / color rule.