Developing a FileMaker Database without scripting is like rowing a row boat with only one oar.
The "allow creation..." option makes it possible to use the relationship to add related records. It does not make it an automatic process. If you had a portal to the table of values, "allow creation" would allow you to add a new record and thus a new value to your list by entering data into a field of the blank "add row" that appears in a portal when you enable that relationship option. Such a portal could be hidden inside a popover.
But here's a simple script for adding a related record. While the script changes layouts, the user does not see the change take place:
Set Variable [$ID ; LayoutTable::PrimaryKey ]
Go to Layout ["relatedtable" (relatedtable) ]
Set field [RelatedTable::ForeignKey ; $ID ]
Go to Layout [Original Layout ]
You may find the following two demo files to be helpful. The first illustrates 10 different ways to set up a conditional value list. The second explores options for selecting values when a value list, conditional or otherwise, isn't enough to get the job done.
They are free to download.
Adventures in FileMaking #1 - Conditional Value Lists (includes details on how to set up a basic field based value list)
Adventures in FileMaking #2 - Enhanced Value Selection (what to do when a simple value list won't cut it.)