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 line counts as one entry. Words separated by a space 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. If you allow them to submit more than once, they stay on the same page once they press 'Submit', letting them type in more answers.