how to add the text fields into the dropdown like as you can see into the image in the personal details there are the text fields like first name and other text fields ?

Not really sure what you are referring to -  by dropdown, are you referring to the collapsible section with a blue bar? 

I assume you're trying to add different components into a Box Layout and calling it a Dropdown since it collapses/expands. 

Box Layout - Container for other components (Can collapse, and 'drop down' when you click on it)
Dropdown - Component which lets you select item/items from a list
Text Field - Free Input

If I'm right and this is what you're trying to achieve, you can add various components into the contents of a Box Layout -