This article explains how to extract parts and create multi-measure rests in the Notion iPad app:
1. You must have more than one instrument in your score to use multi-measure rests.
2. You must first extract a part. To do this, click on the gearwheel and go to View>Dynamic Parts... Choose a part to extract.
3. Click on the gearwheel and go to Parts Settings.
4. Scroll down to "Multi-measure rests" and press the + sign to choose a number. As an example, if you choose 2 measures, that means that anything that exceeds 2 measures of rest will be converted to a multi-measure rest.