Mentimeter has two question types where the audience can provide free-text input: Word Cloud and Open Ended question. The key takeaway for using these question types is to remember to give the audience a bit more time to answer - it takes longer to type than to choose from predefined options.
When participants input words to a Word Cloud, every text field counts as one entry. Words separated by a space such as 'Swan Lake' will therefore be recognized as one single entry. You can set the number of entries available for voters when creating the question.
Open Ended questions
To answer an Open Ended question, participants input the textual responses they want to give. The response allows for 200 characters, but if they have multiple questions or ideas, you can allow them to vote more than once from the same device. In that case, they stay on the same page once they press 'Submit', letting them type in more answers.