Hi Dawid,
Thank you for contacting Syncfusion support.
Currently we do not have support to enter text to filter in multiselect mode of SfComboBox, but we have implemented a feature for this support in SfComboBox which will be included in our Volume 3 release which is scheduled for the end of September.
Please let us know if you have any further queries.
Regards,
Nikhilesh O