Hi BAO,
Thank you for contacting Syncfusion support.
In SfTextBoxExt, we have filtered the items to the SuggestionBox based on the text entered in it which in turn omits the space entered in it. Therefore, empty string cannot be added as item in SfTextBoxExt. Can you please share the purpose of adding empty string to the control? Based on your update, we will update further in this.
Regards,
Keerthana J