Sumo's awesome dev team built the integration to connect with ConvertKit! We've tested it and it works great.
If you aren't already familiar with them, Sumo offers the most powerful marketing tools online—and they are all free to start.
Our favorite Sumo tools are:
Activate Sumo + ConvertKit
Once you have Sumo setup you can go into the Services section to connect to ConvertKit. Just select us from the list and enter your API key. From there, select which ConvertKit Form you'd like your Subscribers to go to.