You can either populate a dropdown from another data room in Sysero or from an external source, this article will use the internal option as an example:

1. Create a new data action in Admin > Data Actions.
2. Set the type to SQL.
3. Set the Connection String, using the same one you used in the web.config when accessing Sysero.
4. Find out the fieldnames from the data room you want to use data from e.g. ItemName and TextField1.  Also note the ApplicationID in the URL.
5. Example SQL command will be something like this "SELECT ItemName, TextField1 FROM Claims WHERE ISNULL(Archived,0) = 0 AND ApplicationID=1 ORDER BY ItemName"
6. Dropdowns will be populated with either one or two output mappings, if only one output mapping it added it will be both display value and stored value. 
7. Output mappings should be set to "No Output" as not populating other fields.
8. The first output mapping should be the display one e.g. ItemName, the second one should have the Hidden checkbox checked and in this example may be TextField1.  Sometimes you may decide to store the unique ID, the field for which in the Claims table is ID.
9. Once the Data Action is created, go to the Form, set field type to Data Action, go to Data Action Settings and set display type to Dropdown and select the data action to populate it from.