Pre-fetching an Eloquent model from a relationship Select to use in Closures

use Filament\Forms\Components;

//[omitted for brevity...]

public static funciton form(Form $form): Form {
  return $form
    ->schema([
      /* [other components...] */
      Components\Select::make('related_model_id')
        ->relationship('relationship_name', 'titleAttribute'),
      Components\TextInput::make('some_field')
        ->afterStateUpdated(fn(/* what to put here?*/) => $somehow_related_model_instance->someMethod() /*or property, or whatever*/)   
    ])
    //[...]
    ;
}

use Filament\Forms\Components;

//[omitted for brevity...]

public static funciton form(Form $form): Form {
  return $form
    ->schema([
      /* [other components...] */
      Components\Select::make('related_model_id')
        ->relationship('relationship_name', 'titleAttribute'),
      Components\TextInput::make('some_field')
        ->afterStateUpdated(fn(/* what to put here?*/) => $somehow_related_model_instance->someMethod() /*or property, or whatever*/)   
    ])
    //[...]
    ;
}