There is not much information on the Listary website on how to add custom search engines, but I was able to add Ecosia in the Listary options, just follow this image:
After clicking on apply and OK, and just by typing the keyword
eco followed by a space, Listary will appear like this:
In suggestion provider I didn’t know what to put in its respective field, so in the end I decided to leave Google as the suggestion provider.
To see that all of the above works correctly, I did a search on ecosia.org, then I did the same search with Listary and… TaDa! shows the same results and the counter of planted trees also increases its number as if you searched only through the Ecosia website.
I hope my answer has helped you
P.S. By the way, use this image to appear in the list of Ecosia results through Listary, just click on the folder icon you see in the first image and select the image, that’s all.