You can edit the settings for dropdown or other multiple choice questions in your form, so that the User completing the form can add new options to the multiple choices, and/or save those new additions for future use in other forms using the same template.
To do this, in the Web Portal go to Forms > Existing Templates > Select the Edit Arrow for the Template > Form Builder > Select the drop down field you would like to edit > Select Allow other and Save other.
To save the changes ensure you Save Draft and Publish / Deploy Template.
When Allow Other has been enabled it add in an additional text field below the dropdown/checkbox.
If you would like to add items into a dropdown permanently, or if you would like to remove or tidy up the items, you are also able to edit them in the Form Builder.