In the Form Builder, you can create Form Events. This can be done to make an invisible section become visible and vice versa, but it can also auto-populate another field in the form with a set value answer that you have created in the multiple choice field's Events section.
Go to the Form Builder in the Web Portal. Go to Forms > Existing Templates > select the edit arrow for the desired form > Form Builder.
To view the set values for a Form event, first locate and select the multiple choice field that you need to change the values for > select the Events tab > select View Existing Events. This will show you what events are current set up, the answers they are connected to, and the Object name of the field that the event values will auto-populate.
You can see the values in the Event column. If any are incorrect or you want to delete any, you can select the checkbox(es) and then select Delete.
To add a new value to one of the multiple choice answers, you can close that View Events window and then in the Form Settings under the Events tab again, you can create new set value answers.
Select the answer you would like to create the Event for, then in the Set Value field, add the value that you would like to have appear automatically in a field. Once that is done, you can drag the Set Value window to the field where you would like the answer to be auto-populated.
Remember to Save Draft and Deploy / Publish Template before leaving the Form Builder screen, to save these changes to the form.