Validation of a field inside multifield - Dialog Touch UI

Hi, 

Does anyone know how to validate a field which is inside the multifield in touch ui using validator.register("foundation.validation.validator"). 

What should be the selector used here to pick the field inside multifield. The tricky part is that there can multiple fields as it is inside a multifield. Lemme know if anyone has done it using this way using a selector. If yes, Kindly let me know which selector should be used. 

I am specifically looking to validate a tag field but example of a simple text field will also help.

Thanks and Regards,

Vijay