You do need to use hot water, but you don't need a whole washing machine full. Turn on your washer making sure the water is set to "hot". Add your soap and let it fill up a few inches, then shut the washer off. Stir the water with a broom handle or old wooden spoon and let it sit for 10 minutes. Your soap should be dissolved by now. Turn the washer's water temp to what you usually use and proceed as usual.
If this isn't practical you can fill a large container with hot water and dissolve the soap in that, then add that to your washload. The soap won't dissolve if you didn't grate the pieces finely enough. Hope that helps.