Allen said it's difficult to determine where the roots are if the plant has been jabbed in through the moss. She recommended using an all-purpose potting soil and adding polymer granules. The granules retain moisture. And that's important with a hanging basket. Small summer annuals work well in hanging baskets.
Herbs are another option.
Even lettuce could work, if planted in the early spring. Plants have various sun-shade requirements, so have your spot picked out before you pick out your plants. Hanging baskets in especially sunny areas do better if they have some shade. Or you could spend much more. Some hanging plants are sold ready to go.