Default behaviour

When creating a Microsoft Flow, one of the many, many possible triggers is For a selected item: 

This allows an Office 365 user to select an item in a list or a file in SharePoint library and start the flow manually after entering some of the required information. Currently there is no UI option to set the user inputs as optional. As long as any of the input fields is missing, the user cannot start the flow:

Change the required input field to optional

1. Navigate to Flow and export your flow as a zip package:

2. Extract the package and locate definition.json file.
3. You can beautify the json text using a json formatter, e.g. 
4. In the beginning of the file, after

you should see the required fields:


5. Remove the fields which you want to make optional.
6. Save the file and package the Flow again as .zip
7. Import the Flow.

8. Now your field became optional:

See Also

Microsoft Flow
Error handling in Microsoft Flow