How can I pass a constant value for 1 binding in multi-binding?

If your value is simply a string, you can specify it as a constant in the Source property of a binding. If it is any other primitive data type, you need to define a static resource and reference this.

Define the sys namespace in the root of the XAML to point to System in mscorlib, and the following should work:

<TextBlock>  <TextBlock.Resources>    <sys:Int32 x:Key="fixedValue">123</sys:Int32>  </TextBlock.Resources>  <TextBlock.Text>    <MultiBinding Converter="{StaticResource myConverter}">      <Binding Path="myFirst.Value" />      <Binding Source="{StaticResource fixedValue}" />    </MultiBinding>  </TextBlock.Text></TextBlock>

Or, combining the two answers above:

<MultiBinding Converter="{StaticResource ScalingConverter}">    <Binding>        <Binding.Source>            <sys:Double>0.5</sys:Double>        </Binding.Source>    </Binding>    <Binding ElementName="TC" Path="ActualWidth" /></MultiBinding>

Which provides the right type without the Resources kludge.

I don't quite follow the question but there are two options:

Put the line <Binding Source="123" /> in your multibinding will pass 123 as a bound value to your converter.

Put ConverterParameter="123" in your MultiBinding:

<MultiBinding Converter="{StaticResource conv}" ConverterParameter="123">