You can't change in-game username. But as far as I know you have to use forum name and password in /linkforumaccount command. So try using /linkforumaccount UsmanMughal forumPassword
Did you create this forum account using your in-game account name ("UsmanMughal" to be exact)? If yes, could you provide it to see if it contains some weird characters that could be conflicting with the script itself?
I think to solve this I have to remove space in my forum username, I need to know how to remove it?
As far as I know, your username cannot be changed, unfortunately, but I guess is possible in extreme cases and is only done by forum Administrators. For the meantime, try linking it without leaving the space and using uppercase, type it all in lowercase, meaning if your username is "Cobra Commander" it should be as follows: /linkforumaccount cobracommander yourForumPassword and see what if works. I believe it doesn't need to be changed in order to link it. Try what I said earlier, hopefully it works out.
Try what brand said, but its probably impossible because there is restriction in username (uppercase and lowercase)If it didnt work you need adminstrator/l5 to change your username. Use account retrieval board, its the right board to request that.