Hi @Michael_Soprano ,
Just to make things clear, web sdk calls always have ECID inside identityMap. The documentation you shared talks about method to access the ECID from the identityMap of the event and pass it in any other XDM field of your choice, e.g. a custom field that you created in the schema to collect EXID out of identityMap. If you don’t have such requirement you don’t need to implement it as web sdk will always have ECID inside identityMap on every event.
Now, incase you have a requirement of reading the ECID from the identityMap and pass it to a custom field using the method suggested in the documentation you share, you can start the data prep with an empty JSON object like {} and it will enable to Next button and let you proceed with data prep configuration. One you are in the data prep screen you can simply add xdm.identityMap.ECID[0].id in the source field and map it to target field of your choice.
Cheers!