Hey
@Duong Quoc Huy,
The invitation process has recently changed. Now you will be put into a queue to get your channel and videos reviewed by a Freedom! Staff Member. It will take around 2-3 days, lately longer, to get reviewed. After that I believe you are contacted through Email for a quick chat with a Freedom! Staff Member explaining everything to you and then "Sending you the Learn More button".
As the process has recently taken longer quite often, please contact support if it's been more than a week and they will sort it out for you.