In the end event you can set a return type. Set this to be the same type as your FullName variable, and set the return value to be your FullName variable.
https://docs.mendix.com/refguide/end-event/
I hope this helps.
If you retrieved a list you can:
- select first instead of all to convert to an object
-iterate over the list to find the object you need
Then you can either return the object or store it in a variable by setting it to string and using {yourobject}/FullName as expression and then return the variable.
You can rightclick on the item you want to return and then click on 'use as return value'