SalesForce Marketing Cloud (also known as ExactTarget) enables clients to create re-usable Content Blocks or add Content on an Ad-Hoc / Per Template basis.
The following guide focuses on creating a re-usable Content Block which can then be added to all future templates.
1. Create your New Content Block
From within the SalesForce Marketing Cloud Email Studio, go to Content > Content Builder to start the process of creating a Custom 'Content Block'.
One the relevant section within SalesForce Marketing Cloud loads, simply click on the Create button.
And from the relevant Drop Down, select 'Content Block' and select 'HTML' as a type.
2. Insert your Embed Code
Once you have selected HTML as a type, you will now be presented with the opportunity to add your relevant Email Plugin Embed code.
Within the Content Builder, simply select the 'HTML Editor' tab and paste in the code.
You can now Name and Save your Content Block.
3. Add your Content Block within your Email
Once you have created your Content Block, it should be available as a Content type whenever you are editing an Email Template within SalesForce Marketing Cloud.
Content Types are available within your Email Editor under the 'Content' tab which is shown on the Left Hand side of the screen.