In my project on AEM Forms, when user selects enter new scheme, a Pop Up Text Box should appear that takes scheme number and after hitting OK on pop up, it should disappear and that value should be populated in one of the original form fields.
I did not come across Pop Up Text Box feature of forms.
Inject popup dialog html using a client library attached to the form. Reference code is shared @ https://jsfiddle.net/nepqga53/5/ you would need bootstrap and jquery as dependencies for this. Call prepareSchemePopupDialog() function on change event of select field to 'Enter new scheme'