How can we enter an ITIN instead of a SSN?

Then you do not have an ITIN.

IRS website for an ITIN - https://www.irs.gov/individuals/international-taxpayers/taxpayer-identification-numbers-tin#

ITIN

An ITIN, or Individual Taxpayer Identification Number, is a tax processing number only available for certain nonresident and resident aliens, their spouses, and dependents who cannot get a Social Security Number (SSN). It is a 9-digit number, beginning with the number "9", formatted like an SSN (NNN-NN-NNNN).

If you were sent a 6 digit number from the IRS then that is Not an ITIN.  That more than likely is an Identity Protection PIN (IP PIN) because you reported an identity theft to the IRS and were sent the IP PIN to be entered on your tax return to prove the return being filed was from you.

To enter, edit or delete a 6 digit IP PIN

• Click on Federal Taxes (Personal using Home and Business)
• Click on Other Tax Situations
• Scroll down to Other Return Info
• On Identity Protection PIN, click the start button

Or enter ip pin in the Search box located in the upper right of the program screen. Click on Jump to ip pin