A few days ago I implemented a custom field which should have 2 dropdowns which are depending on each other (here you can find the whole implementation, in this example I will just show you a simple text field).
So I checked all possible solutions and I decided to use the Iframe type for that. This is a default field provided by Sitecore. So in the source input on the template you need to provide the url for your iframe. In my case it will be a Controller: