You can setup a multiple choice question with an Other dialog box and have a hidden text entry field show up, using Skip logic.
1. Create your multiple choice question with one of your answers being "Other"
2. Create your follow up question by adding a single line text box question. Click edit to change the question text to something like, "Enter your Other selection here"
3. On the Edit Fields & Questions Screen, click the grey branching ion next to the Single line text box question.
4. Make the dependent question your multiple choice question.
5. Set the answer to trigger as "Other - Selected". Save the result.
You will now see the skip logic icon turn green and you will see the hidden text box show up when your customer selects "Other".
This will show up on the following screen in tablet mode or automatically appear in portrait mode.