The following applies if you are using a version of the software before Update 40. For the latest information, see Create a widget.
Before Update 40, in the URL tab, when completing the URL to return the required data, variables should always use upper case. They should also be wrapped in single quotes ' for all properties except enumerated lists. For example:
Tags('{{WIDGET_TAGLIST_ID}}')
Tags?$filter=Name eq '{{WIDGET_DETAILS_NAME}}'
Tags?$filter=Tag_Status eq {{WIDGET_DETAILS_TAG_STATUS}}