The matter is broad in scope. She may say the words that suit her (i.e., the feminine form) and say Ana amatuka (I am Your female slave), or she may use the wording of the du’a’ that is narrated, because it is describing the person who is speaking, and may apply to both male and female.
Shaykh al-Islam Ibn Taymiyah was asked about a woman who heard in the hadeeth “Allaahumma innee ‘abduka wa’bnu ‘abdika, naasiyati bi yadika (O Allaah, I am Your slave, son of Your slave, my forelock is in Your hand)” and she persisted in using this wording. It was said to her: Say “Allaahumma innee ‘amatuka bint amatika … (O Allaah, I am Your female slave, daughter of Your female slave…)” but she insisted on reciting it in the wording that she had been using. Was she right or not?
Rather she should say “Allaahumma inni amatuka bint ‘abdika ibn amatika (O Allaah, I am Your female slave, daughter of Your (male) slave, son of Your female slave). That is more appropriate. But if she says ‘abduka ibn ‘abdika, then it is acceptable in Arabic usage, like the word zawj (spouse) which may be applied to both male and female. End quote.
Majmoo’ Fataawa Ibn Taymiyah (2/177).
Ibn Hajar al-Haythami (may Allaah have mercy on him) said in al-Fataawa al-Kubra (5/342): A woman may say in sayyid al-istighfaar and similar du’aa’s: Wa ana amatuka bint amatika (I am Your female slave daughter of Yourfemale slave) or bint ‘abdika (daughter of your male slave). But if she says ana ‘abduka (lit. I am Your (male) slave), that is acceptable in Arabic usage, like the word shakhs (person) which may be used for both male and female. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked about this and he replied: The matter is broad in scope, in sha Allaah, but it is better for her to say Allaahumma inni amatuka wa’bnatu ‘abdika wa’bnatu amatika (O Allaah, I am Your female slave, daughter of Your male slave, daughter of Your female slave), and so on. This is more appropriate and more befitting for her. But if she says the wording that is narrated in the hadeeth, that does not matter in sha Allah, because although she is amah she is also ‘abd, one of the slaves of Allaah (‘ibaad Allaah). End quote.
Majmoo’ Fataawa Ibn Baaz (6/76).
And Allaah knows best.