How can I add a father to a child when the mother and father were not married?
You add the child to the mother and father as if they were married.
But you don't add a marriage date for the mother and father.
Here is the article which covers adding a child. It also mentions what to do when the parents didn't marry.
Thank you very much.
A Child is not sealed to unmarried parent. Instead this knowledge article stands. https://www.familysearch.org/help/helpcenter/article/how-do-i-seal-a-child-to-grandparents
We seal the child to the grandparents. On occasion, you may need to link a person in Family Tree to grandparents or other foster parents for sealings to be performed. This is the easiest to do if you are working from the website, FamilySearch.org, as opposed to the Family Tree app or Family Tree Lite.