You can't make any manual entries in turbotax online. If you are filing as head of household, you are the taxpayer so your name is entered automatically.
I think there is something else going on. Can you explain what you are trying to do?
In the past, my accountant entered the name of my oldest son as the head of household, while still listing him under dependents. So I am trying to replicate what was done last year.
You must be confused about what your accountant did. Perhaps he said YOU were head of household and your son was your qualifying dependent. It is impossible to be a dependent and also head of household.
Am I Head of Household?
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To file as head of household, you must be unmarried, or separated for at least six months; and you must provide care in your home for a qualifying dependent, usually your child; and you must pay more than half of your household expenses.
Start by selecting in the personal interview, where you enter your name and address, that you want to file as head of household. Later, you will add the names of your dependents. As long as at least one of those dependence is a qualifying person for head of household, the program will let you do that. You don't separately list one of the qualifying people. You simply list all of your dependents in the usual dependent section.