<StackPanel>
    <TextBox>
        <TextBox.Style>
            <Style TargetType="{x:Type TextBox}">
                <Setter Property="Height" Value="200" />
            </Style>
        </TextBox.Style>
    </TextBox>
    <ToolBarPanel Orientation="Horizontal">
        <Button Content="Funky stuff" Command="{Binding Query}" CommandParameter="{what to type in here?}" />
    </ToolBarPanel>
</StackPanel>

<StackPanel>
    <TextBox x:Name="myText">
        <TextBox.Style>
            <Style TargetType="{x:Type TextBox}">
                <Setter Property="Height" Value="200" />
            </Style>
        </TextBox.Style>
    </TextBox>
    <ToolBarPanel Orientation="Horizontal">
        <Button Content="Funky stuff"
                Command="{Binding Query}"
                CommandParameter="{Binding Text, ElementName=myText}" />
    </ToolBarPanel>
</StackPanel>

CommanParameter="{Binding Text, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type TextBox}}}"