To assign the main product(s) that you want to sell using this funnel, you will get the options in the checkout step.
Select the Checkout step and click on the settings icon.

Once you click on the checkout step settings, the configuration drawer will appear.

Here, search for your product and select it. And then click on the ‘+ Add Product’ button on the right side.

You will see that the product will be added to the funnel.

Once the product is added, the next thing to do is to design your funnel steps and use the right content in the step pages.