Normally, you will do that at the beginning, if you indicate you are single or separated and have a qualifying dependent, turbotax will ask extra questions to determine if you qualify. However, if you indicate single or married to start, and want to change to HOH, you can do it at any time and your taxes will recalculate.
Either...as it doesn't affect the entries. But it's in the very first part of the interview.
It's weird that the "head of household" is not shown to me even if I have a qualified dependent....
Do you have a qualified dependent and meet the criteria to file HOH?
Am I Head of Household?
https://ttlc.intuit.com/questions/1894553-do-i-qualify-for-head-of-household
https://ttlc.intuit.com/questions/2900097-what-is-a-qualifying-person-for-head-of-household
If you qualify as Head of Household, when you enter your marital status (single or married filing separately) into MyInfo, and then enter your qualifying dependent, TurboTax will offer HOH as your filing status.