There are a couple of options for defining static choices that do not involve Data Model Entities.
The first option is to right-click on the Selection Field and choose Add > Slot to explicitly define the field slots. Slots can be considered a kind of ""sub-field"" and, in case of a Selection Field, they can be marked as Label, Output or both.
The label slot is the one holding the values that are displayed to the user, while the output slot is the one holding the values that are sent to the server.
After defining slots, you just enter the fixed values you want in their Value property, separating them with a pipe character (
The following example shows how to define a selection fields for days of the week that sends to the server a number for each day.
If you need a more complex logic for defining static values, another option is to use a Script Unit to feed the values to the Selection Field, by returning them as arrays (this is why your attempt with link coupling did not work - multiple values must be passed as arrays).