Visa Direct does not use any IAT number for sending or receiving money. Visa Direct transaction takes place based on the 16-digit sender and recipient PAN. Please contact your issuing bank to get the IAT/ICA number.
Please review the Visa Direct documentation using this link. https://developer.visa.com/capabilities/visa_direct/docs
In particular, see the top few paragraphs that are shaded in yellow that discuss the requirements to use Visa Direct.
After having read the documentation, if you meet those requirements, then please click through the link in those paragraphs to the Visa Direct Program Implementation Questionnaire, fill it out, and submit to the email address listed on that form. Please let us know if you have other questions.
Visa Direct APIs are available for all developers to try out in Sandbox environment in Visa Developer Platform.
Please take a look at the Getting Started Guide to create a new project and add Visa Direct APIs to your project.