JSON format required. Relation Configuration. Define how the interface should fetch the data and how PHP should generate the links. Full example (multiple project selection):

{
        "multiple": true,
        "endpoint": "api/entries/project",
        "resource": "project",
        "references": "entries",
        "references_key": "entry_id"
    }

Available keys:

endpoint: (Required) The API endpoint used for searching (e.g., "api/pages", "api/entries/project").
resource: (Required) The singular handle of the content type (e.g., "page", "user", "article"). Used for generating public links.
multiple: (Required) true to allow multiple selection, false for single selection.
references: (Optional) The target SQL table name (e.g., "projects"). Used to create foreign key constraints.
references_key: (Optional) The primary key column name (e.g., "project_id"). Default: "entry_id".
type_key: (Optional) Enables polymorphic relations by defining the field containing the resource type (e.g., "type"). When defined, the autocomplete stores both an ID and its associated type.
input_name_id: (Optional, polymorphic only) Name of the hidden input used to store the selected item ID (e.g., "fields[target_id]").
input_name_type: (Optional, polymorphic only) Name of the hidden input used to store the selected item type (e.g., "fields[target_type]").
on_delete: (Optional) SQL action when the target is deleted. Values: SET NULL (default) or CASCADE.

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