Login to Learnyst as Admin and go to Website builder. In website builder you can add your custom code by adding a custom code section to your landing page from the side bar. Follow the below steps to do the same.
In side bar go to sections and add a custom code section.
In custom code section click on Edit HTML.
Replace the existing code with your Custom Code and click on Save.
Once you save the custom code click on Save button on the side bar to save the settings and Publish it.