Learn more about this change here.
For help regarding Aternos, we recommend you to visit our support center.
You can apply the datapacks when you generate a new world in the worlds section.
I found it, thanks for the help 
just took a bit to figure out, my apologies.
For those who are still in need of help, when you generate a new world, and the server is in the newest snapshot, you can click on datapacks when you scroll down on the generation screen, where the seed is and everything, and you can click on both the options.
Adriel, no need to be rude. I'm suffering the same issue, where I am struggling to download the 1.20 snapshot datapack for my server. I have no clue on how to get the materials that the actual 1.20 snapshot provides onto the server, despite it being in the new snapshot. It doesn't have any of the new update material when I log onto the world, so I am unsure of how I can add the datapacks of bundles and 1.20 experimental snapshot onto the world.
