In this video I go through how you can update a Model-driven form that launched the Custom Page from the custom page. There are two methods of this and both work slightly different depending on how you launched the custom page.
SmartButtons – https://github.com/scottdurow/RibbonWorkbench/releases
Code snippets – https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/navigate-to-custom-page-examples
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