Any version of Xamarin.Forms, the iOS version of the application crashes when attempting to apply a custom font "parameter descriptor cannot be null"
With this resource defined, and the OpenSans-Bold.ttf is in the Resources directory, marked as bundle resource...
<OnPlatform x:TypeArguments="x:String" x:Key="BoldFont">
<On Platform="Android" Value="OpenSans-Bold.ttf#Open Sans" />
<On Platform="iOS" Value="OpenSans-Bold" />
</OnPlatform>
And using this code for the style (created from the Telerik login view template, with some modifications):
<Style x:Key="LoginEntryStyle"
TargetType="telerikInput:RadEntry">
<Setter Property="TextColor"
Value="{StaticResource DarkTextColor}" />
<Setter Property="WatermarkTextColor"
Value="{StaticResource LightBlueColor}" />
<Setter Property="FontSize"
Value="14" />
<Setter Property="FontFamily"
Value="{StaticResource BoldFont}" />
<Setter Property="HeightRequest"
Value="48" />
<Setter Property="Margin"
Value="0,0,0,16" />
<Setter Property="BorderStyle">
<Setter.Value>
<telerikInput:BorderStyle BorderColor="{StaticResource LightBlueColor}"
BorderThickness="1"
CornerRadius="8" />
</Setter.Value>
</Setter>
</Style>