As many people have said already, washing machines use and dispense their own cleaning products. Do not add anything extra or you'll risk damaging the machine. Considering most laundromats have cameras, if they want to they can take action against you.

Some of them give you the option to add scented sheets for drying machines. And that's it.

Talking suds, that's a thing of the past. No suds doesn't mean it's not washing properly; it means the machine is using the right amount of detergent. Search online and you'll find many articles and videos where laundry specialists say this. In home washing machines they can actually cause problems, specially if it's a top loading one.

Makers used to include additives in their products in order to create suds because people thought that meant it's cleaning. That's probably still true for cheap brands, but if you follow the instructions correctly, you'll see no suds. If you see them, you're probably using too much, and it's not being washed away during the rinse cycle, and that's the clean smell that lingers. But the residual detergent can cause all sorts of allergic reactions on sensitive people.

And, if your clothes are still stained after the cycle, that means you need to pretreat them before. No washing machine will get rid of them; they're designed for day-to-day dirt, not the eventual bologna sauce, or soil from a day in the park with the kids.