Editing a Relationship

I have a form that contains a related record for which I want to submit data at the same time. For this I have set up:
Forms\Components\Section::make()
    ->relationship('foo')
    ->schema([...]);

I need to perform a mutation to add extra data when saving, but I can't find a way to do this on the relationship. Does anyone have any suggested on how I can achieve this?
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