This article details how custom fields used by the survey API are to be combined with placeholders to insert the associated custom field information into a Standard Response.
The first thing to note is that the %SURVEY_SUBMITFIELD_field-name-variable% placeholder requires a variable component at the end of it (highlighted in red). This variable takes the name of the custom API field whose associated data you wish to reference. If you have a list of custom API fields and their associated data as per the example in Figure 1, you would reference each custom API field's data with the %SURVEY_SUBMITFIELD% placeholder as follows:
Figure 1 - Custom API fields and their associated data