In an Avalonia window there is a ListBox with an ItemTemplate. In this ItemTemplate the value of the item should be converted, using a ConverterParameter.
So far so good. But the value for the parameter is dynamic and should be taken either from a TextBox, or from the view model.

Try #1 – take converter parameter value from view model property:

<ListBox ItemsSource="{Binding SomeElements}" Width="600">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Converter={StaticResource MagicConverter},
                ConverterParameter={Binding RelativeSource={RelativeSource AncestorType=views:MainWindow},
                Path=ViewModel.PropertyInViewModel}}" />
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

Try #2 – take converter parameter value directly from TextBox:

<ListBox ItemsSource="{Binding SomeElements}" Width="600">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding
                Converter={StaticResource MagicConverter},
                ConverterParameter={Binding ElementName=DynamicInputTextBox, Path=Text}}" />
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

This both works, but the converter gets an Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindingExtension object as parameter.

Any ideas on how to solve this?