I am not sure I fully understand. If you mean that you want to fill the dropdown with a set of options, then set it to show a particular option, and then send a message as if the user had selected that option then you can use two inject nodes. Set the first to fire after 0.1 seconds and configure it to send msg.options containing the selection options, and configure a second inject node to fire at 0.2 seconds which selects the required option. If the dropdown is configured to pass through messages then the second message will also be sent on from the node.
Example:
[{"id":"833750b8970624cb","type":"ui_dropdown","z":"bdd7be38.d3b55","name":"","label":"","tooltip":"","place":"Select option","group":"903a6ab8.4f4ca8","order":5,"width":0,"height":0,"passthru":true,"multiple":false,"options":[{"label":"","value":"","type":"str"}],"payload":"","topic":"topic","topicType":"msg","className":"","x":1110,"y":660,"wires":[["d48c162f34eca317"]]},{"id":"85600131b7280b2d","type":"inject","z":"bdd7be38.d3b55","name":"","props":[{"p":"options","v":"[\"A\",\"B\"]","vt":"json"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":true,"onceDelay":0.1,"topic":"","x":920,"y":620,"wires":[["833750b8970624cb"]]},{"id":"d48c162f34eca317","type":"debug","z":"bdd7be38.d3b55","name":"debug 2518","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":1260,"y":660,"wires":[]},{"id":"bcd98f5e7c2cd90a","type":"inject","z":"bdd7be38.d3b55","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":true,"onceDelay":"0.2","topic":"","payload":"B","payloadType":"str","x":930,"y":700,"wires":[["833750b8970624cb"]]},{"id":"903a6ab8.4f4ca8","type":"ui_group","name":"Home","tab":"e2e6f4f5.56f91","order":1,"disp":false,"width":"6","collapse":false,"className":""},{"id":"e2e6f4f5.56f91","type":"ui_tab","name":"Home","icon":"dashboard","order":3,"disabled":false,"hidden":false}]