Hi Steven,
I am afraid that there is no short example code I can give you to make what you want (in your case, request datas from an external database)
There are two possibilities (maybe more...) I see to make what you want:
The first is exactly what Michal (thanks for the documentation reference by the way :-) ) told in his post. Unfortunately, even though you want to avoid developing custom form controls / web parts, this is still the most straightforward way to achieve this:
developing form controls. From the example described in this link, instead of adding items to the dropdownlist statically (see the method
EnsureItems), you could databind your dropdownlist to a dataset which request datas from your external server (you will have to define your external database connection string in the web.config), using the standard ASP.NET ways of querying data. I don't know if it is possible using Kentico API to use another connection string...
The second, and I won't describe it in detail here as it is outside of Kentico scope, is
configuring a linked server so you can use directly in Kentico a sql query using that format:
SELECT * FROM ServerName.DbName.SchemaName.TableName. However, I am not 100% certain that it will work inside Kentico.
I hope it will help you.
Regards,
Nicolas