In general: accessing the fields is only through advanced JSON
There are a number of fields that are attached to a list set through a gantt chart like UI i.e. Target start, Target due date etc. However, the above fields aren't actually attached to the issue, so probably require integration with Portfolio for Jira's API
It should be possible to select and edit Portfolio fields (like Team, Target Start, Target End ) in Automation for JIRA.
While it is possible to use the Edit Issue action's advanced section (i.e. the one available by clicking on the More options button), due to the lack of support to the Target Start and Target End fields, if you attempt to use any date functions (such as plusMonths, minusMinutes) the automation rule will fail due to a parsing failure.