You don't have to. You can leave it blank if you want to. It's just for verification if you forget your password or if Thang wants to verify you and contact you as a proof that you are the owner of the account. Nevertheless, you can leave it blank.
Just a note though, I have tried this one wherein I input a different number which I can remember for reasons that when something happens with my account, there is still a possibility that I can retrieve it due to the number as a proof that the account is mine. I pressed continue, and it worked out just fine.
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You don't have to. You can leave it blank if you want to. It's just for verification if you forget your password or if Thang wants to verify you and contact you as a proof that you are the owner of the account. Nevertheless, you can leave it blank.
Just a note though, I have tried this one wherein I input a different number which I can remember for reasons that when something happens with my account, there is still a possibility that I can retrieve it due to the number as a proof that the account is mine. I pressed continue, and it worked out just fine.