You can add an optional “Other” text box to Multiple Choice Questions by checking the option in the question settings. You will find the option at the bottom of the question choices in the “Rows” settings panel. When you check the option, the input field will be enabled. By default, the placeholder text in the input will be “Other”, however, you can change this to anything you want.
Any answers that are typed into the “Other” text box by your participants will then appear in your results. You will be able to see them in the Individual Response view, in the Report section using the “Data Table” chart type as well as in the Excel/CSV Export.