The “Submit” button is included by default to every Form. However, if you send a link of a Form to a respondent from a private folder, which did not allowed everyone to respond. Then, the “Submit” button will not be shown to that respondent and hence, he/she will not be able to respond. The submit button will be shown only to those who are allowed to respond. This option can be done from Form-Share-Settings as shown in the following screenshots.