Thank you for posting in our community! 

In general, there are several key points here.

1. Every component to be used in a reactive form should implement ControlValueAccessor Interface. IgxDropDown does not. IgxCombo does it out-of-the-box.
2. The IgxDropDown does not contain an input on its own. 

So in order to implement a somewhat similar to the IgxCombo component, it will basically require creating a custom component utilizing an input and IgxDropDown for example plus implementing ControlValueAccessor Interface. 

Why is that you wish to use the IgxDropDown instead of the IgxCombo? The second is built upon the first one and provides more functionality and flexibility. Is it because you require a single selection only or else? If you are after simplified combo version with single selection, it could be considered a new feature request. I suggest you to “vote” for implementing such a “Single Select” component. When planning new releases, our Product Management team will look at the most popular features and use your feedback to prioritize upcoming work. 

I am looking forward more feedback from you!

Related samples:

• Reactive Forms Integration
• IgxCombo
• IgxDropDown

You will find such an implementation here: https://stackblitz.com/edit/combo-single-selection-ig. Optionally it should be possible to implement ControlValueAccessor as well, in order to use it in a reactive form.