You enter direct deposit information into your return when you file your return & receive a refund. If you have not filed yet, you enter your information when you go through the File section.
If you filed a return & it has been accepted, you cannot add or makes changes to the refund information, the IRS would not receive it.
From the IRS:
What if my direct deposit information has changed or I want to add it for the first time?
On March 30, the I.R.S. said on its website that it would build a portal where people can update their information “in the coming weeks.”