With Survey123 for ArcGIS you can configure smart forms to capture signatures. This feature is supported in the Survey123 field app, allowing you to add a handy signature pad. Once a signature is captured, it will be stored as an image attachment in ArcGIS.
To add a signature question into your form, you need to use Survey123 Connect or ArcGIS and author your survey using an XLSForm file. Add an image question and set the appearance to signature as shown in the screenshot below.
You can add one or more signature questions into your form. You can in fact even add signature questions within repeats.