Thanks for using Syncfusion product.
We don’t have any styles for MS Button, so no need for BasedOn property. You can apply the style for a Button, as per the below code it will works.
Please refer the below code snippet,
<Style TargetType="Button" x:Key="CustomButtonStyle" >
<Setter Property="Padding" Value="20, 10"/>
<Setter Property="Background" Value="Red"/>
<Setter Property="HorizontalAlignment" Value="Left"/>
Please refer the below screen shot for your reference,
Let us know whether this helps also if you need any further assistance on this.