From the instructions (which always should be your main fall back):
"Married Persons Filing Separately
Generally, married persons must file a joint return to claim
the credit. If your filing status is married filing separately
and all of the following apply, you are considered
unmarried for purposes of claiming the credit on Form
2441.
• You lived apart from your spouse during the last 6
months of 2018.
• Your home was the qualifying person's main home for
more than half of 2018.
• You paid more than half of the cost of keeping up that
home for 2018.
If you meet all ..."
See page 2 of https://www.irs.gov/pub/irs-pdf/i2441.pdf