You can add the drop-down menu to the field and set the values that will be available for the Verification operator to choose from by using the Data Set and Allowed Values option in the Field Properties or by using the IField::Suggest method in the rule script.

By using the Data Set functionality.

• Open your Document Definition Properties, go to the Data Set tab and Add a new Data Set.
• Add the Primary Key column to the Data Set. 
• Add the column that will contain values.
• Choose your created Data Set and click View. Add the values.
• Confirm the Data Set settings:

• Open the Field properties and go to the Data tab. Click the Edit button near the Validation and set up the Allowed Values using the following properties:

By using this setup, you make the Verification Station allow only specified values for a field. These values will be available when editing the text and suggested as auto-fill options, as well as in the form of the items in the drop-down menu. You can also use the option Treat validation error as warning to allow other than specified values.

By using the IField::Suggest method.

//C# .Net
Context.Field("test_field").Suggest("test1");
Context.Field("test_field").Suggest("test2");
Context.Field("test_field").Suggest("test3");
Context.Field("test_field").Suggest("test4");

After adding this rule, specified values will be suggested to the Verification operator from the drop-down menu the same way as above.

To set up the default value, you can specify the Default Value in the Data tab of Field Properties.