1) Create a D-A-DB-Name instance and map it to your external database

2) Go to Data Model => Database Table class mapping landing page

3) Click on new database table mapping and provide required details, this will create class and properties.

4) Create a report definition on this class to fetch the values you required for dropdown

5) Create a data page with report definition as source

6) Configure this data page as source of dropdown control

How to create a D-A-DB-Name?

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Go to Records explorer .

Records -> SysAdmin -> Database -> Create

Ok Thx all!