I missed putting the referral code while submitting my order for new connection, how can I add that

You cannot is too late
You need to cancel the order and redo it

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If a person forgets to enter the referral code when subscribing, they’re unfortunately out of luck. We cannot retroactively apply a referral code.

source: https://fizz.ca/en/faq/how-do-i-invite-friends-to-join-fizz

Unfortunately it's too late.

When you put your credit card informations, to confirm the sim card and transfert, you need to put the referral there, for Mobil plan.