The IAT/ICA number is not related to the Visa Developer Portal. Please contact your issuing bank to get the IAT/ICA number.
To what I know is that IAT number is assigned to the billing entity. Let’s say if you have to wire the money into someone’s account, the billing company should give you the information like Account number and IAT number. I can’t be very sure on this but usually this is what I have seen.
Since we are a developer platform, not a financial institution, I would recommend you do some research online or follow up with the issuing banks in obtaining the IAT/ICA Number.
However, I will continue to research within and will update you if I find any information.