My Baby's father is Unknown

My eldest daughter is 7 yrs old. Di na kami nag cocomunicate ng father nya after we broke up and then I found out I was preggy last 2014. Her father knew about it and played dumb. last 2019 I got married to her Step-dad. Few weeks ago he opened up about placing his last name on my eldest daughter's birth cert. My question is can we put my husband's name on it? kasi UNKNOWN yung sa father side ng birth cert. ng baby ko. #advicepls #pleasehelp (Picture attached below is my daughter's birth certificate.)