Implement an AllowCustom option in the MultiSelect, which would make possible to select a custom value that is not present in the data.

This option is available in the Kendo UI for Angular MultiSelect:
https://www.telerik.com/kendo-angular-ui/components/dropdowns/api/multiselectcomponent#allowcustom
https://www.telerik.com/kendo-angular-ui/components/dropdowns/multiselect/custom-values