Issue with Survival Island Generator mod

So the Survival Island Generator mod tells us to make a new line in server.properties and write "use-modded-worldtype:island" in it, but aternos does not let you edit the server.properties with writing or something like that. Am i just stupid and missing out on something?

ive tried but i dont think its working. I might just be stupid

when i do that the world looks a little different then in the singleplayer world