Hi David,
Greetings from Syncfusion support.
We would like to inform you that Multiselect with checkbox mode does not allow to type value in the input element. It does allow only to select and deselect values and also you can filter the list of values and this is component’s intended behavior. If you want to enter custom value, we suggest you to set mode as Box or Delimiter which allows to type value in the input.
Regards,
Berly B.C