Adding a custom button in Dyn Portal lookup dialog

Hi All,

Fairly new with the features of Dyn 365 Portal, so need your help in doing a customization. I have this New Case Form where there is a lookup field. User will search for a specific record, if not available then should be able to create a new record by clicking the custom button. Kindly of, how we create records in Dyn365 by clicking the small +New button in the lookup.

Any help would be appreciated.

Regards

Karuna