Setting the custom position of an overlay is not possible when working with the Craft Sync + Prototype tool in Sketch. After syncing your artboards to the prototype, you'll need to set any custom positions for your overlays.
- If you're on InVision V7, read this article: Using Overlays in InVision V7
- If you're on InVision V6, read this article: What are overlays and how do I use them?