Subscribe supports one click UPSELLS for Konnektive, Limelight, Sticky.io, ResponseCRM, and Sublytics.
Setting up the upsell button
Locate the "One Click Upsell" button under the "Plugin Blocks" > "CRM Blocks". Drag the block to the design canvas.
Locate the outer "form" element for the "one click upsell button" and configure it's settings for the CRM, Campaign ID, and Product ID.
Here is an overview of what your project might look like: