You can pass in a string parameter and use that as to address. E.g. call the string parameter as ‘To’ and map it to email to address. In the parent microflow you can pass in the parameter from the object where you are saving email address for a user. E.g. If you are saving user email address in account entity than, you can pass in $Account/EmailAddress as input string parameter.
You can have a look at ‘_Examples’ folder in ‘EmailTemplate’ module.
Hope this helps!