When using a Style key tag, in Unreal RichText, the Style.TargetType property should be used to create an element of that type. This will allow you to create customised inline elements using templates.

If the TargetType is a ContentControl, and the Style tag has content, then this content is set to the ContentControl.Content property. You can use a ContentPresenter to display this.

For example:

<Style x:Key="RndCorner" TargetType="ContentControl">
<Setter Property="Background" Value="Gray"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ContentControl">
<Border Background="{TemplateBinding Background}" CornerRadius="4">
<ContentPresenter/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

<TextBlock noesis:RichText.Text="This is an example of text that has the prompt <RndCorner>Backspace</> inside of it"/>