Make the tube appealing: Make sure the tube is smooth inside, without sharp edges or protrusions. You can also try putting a treat at the end of the tube to encourage the hamster to go through it.
Use positive reinforcement. Use positive reinforcement to reward the hamster when it goes through the tube. Give the hamster a treat or praise when it successfully passes through the tube. This will help to reinforce the behavior and make the hamster more likely to repeat it in the future.
Be patient. It may take some time and patience to train your hamster to go down the tube. Keep trying and rewarding your hamster, and eventually it will learn.