Hi ikomrad! It sounds like you’re on iOS (and likely an iPhone?). If you’re looking at the outline view you can either tap Tags in the perspective bar a second time or tap the sidebar button in the upper left corner to see your list of tags. From here, tapping ‘+’ will add a new tag that you can select as your Forecast tag.
Both macOS and iOS display a “+” sign in the bottom left corner when the tag sidebar is enabled and displayed. Clicking + with the mouse or finger adds a new tag.