One way to do this is with the random word generator option on Smart. You just need to input the names of your students and it will randomly select a student to provide an answer.
Another method is to write the names of your students on Popsicle sticks. Put them in a jar and choose a stick each time you ask a question. Make sure you put the stick back in, so students don't think they're "off the hook" once their names have been drawn.
Just randomly selecting participants will not ensure engagement, however. There are other techniques you can use to help students feel safe to answer. Be very careful of how you respond to answers. Do you use an excited tone to say "that's right!" or "Yes! Good point!"? And then, if the answer is incorrect, do you respond with a very different tone? Most of us do, whether we are aware of it or not. When we celebrate the right answer and downgrade the wrong one, we don't create an environment that encourages risk taking and that says failure is ok--as long as you learn from it!
You can also further the discussion with the Popsicle sticks by asking multiple students to answer. Leave each stick out until you get a variety of answers, and then ask those who responded to debate which answer is the best. By involving only those who answered, the students will not feel "judged" by the rest of the class.
This method is not foolproof; no method ever is. You will still have shy students who cringe at the thought of being called upon to answer. But, when the whole class knows that their names may be selected, there tends to be less pressure. You can also allow students to "pass" while encouraging them to participate at a later time.
If you'd like more speaking and listening activities, check this out.