TextBox control with Content Property

<dev:TextBox Header="TextBox with Content"
                PlaceholderText="PlaceHolder...">
    <dev:TextBox.Content>
        <Button Content="&#xE895;" />
    </dev:TextBox.Content>
</dev:TextBox>

or

<dev:TextBox Header="TextBox with Content"
                PlaceholderText="PlaceHolder...">
    <dev:TextBox.Content>
        <StackPanel Orientation="Horizontal">
            <Button Content="&#xE896;" />
            <Button>
                <FontIcon Glyph="&#xE895;" />
            </Button>
        </StackPanel>
    </dev:TextBox.Content>
</dev:TextBox>

you can run demo and see this feature.