Well, you might check your registration email or any communication related to the game. Also, check the game's FAQ or support page for specific instructions on finding the username. Another option is to contact the game's customer support and ask them for help.
The easiest way could be to search through your account details within the game interface. Failing that, it's worth checking any documentation or notifications you received when you first signed up for the game. If all else fails, reaching out to the game's support team is a good bet.
You could try looking in the game's settings or profile section. Sometimes, it's displayed there clearly.