If you want to use the Material icon, you need to set the FontFamily first.
<ContentPage.Resources>
<OnPlatform x:Key="Material" x:TypeArguments="x:String">
<On Platform="iOS" Value="Material Design Icons" />
<On Platform="Android" Value="materialdesignicons-webfont.ttf#Material Design Icons" />
</OnPlatform>
<Style x:Key="MaterialIcons" TargetType="{x:Type Label}">
<Setter Property="FontFamily" Value="{DynamicResource Material}" />
<Setter Property="FontSize" Value="400" />
<Setter Property="HorizontalOptions" Value="Center" />
<Setter Property="VerticalOptions" Value="Center" />
<Setter Property="FontSize" Value="Large" />
</Style>
</ContentPage.Resources>
<ContentPage.Content>
<StackLayout>
<Label x:Name="label" ></Label>
</StackLayout>
</ContentPage.Content>
And set the Text in code behind.
label.Text = "\ue87e";