In order to achieve the desired result you will have to map a new template for the widget. The template for the Subscribe widget can be found in our SDK - in the archive called WidgetTemplates
C:\Program Files (x86)\Telerik\Sitefinity 5.0\SDK\Content\Resources\WidgetTemplates
( the path is valid for 64 bit OS ).
For more information on mapping external templates to Sitefinity widgets, please check these resources:
Customizing Sitefinity 4 Controls with the ViewMap
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items