All you have to do is stop by the front desk and ask for laundry soap, they will ask how many you need and give them to you for free. We do this every single trip because we stay in studios and they don't leave it in the studios by default, but have even done so the rare times we have been in larger rooms, never had an issue. If you get it from housekeeping, they are supposed to charge you after the first one per trash/towel service or full cleaning, but the front desk isn't supposed to charge you if you ask them.
The nice thing is the soap they give is free of dyes and fragrances, just be aware some resorts give a liquid version, which means if you have any left over you can't bring it in your carry on, but most are powder.