so my requirement is if a user try to type alphanumeric values or numbers ...
Numbers are alphanumeric.
... I need a togglepopup to be opened stating "Only strings are allowed" so that user can know only strings are allowed.
Everything you can type is a string so that prompt wouldn't help anyone.
Anyway, don't do that. Use standard web interface so that your application works as the users expect. For example,
- Set the border of each text field to red and change to black when the field has correct data type in it.
- Use a tooltip to explain the input requirements.
- Disable Confirm Order until all the fields pass the data validation criteria.