Ancestry online trees allow the users to record multiple names. Users can display them by going to the Filter drop down menu above the Facts column on the left-hand side of the profile, and then choosing Name and Gender. If you have entered more than one name, select Alternate Facts as well.
Very often we discover records with a person's married name first, such as a census. Suppose we are starting with the 1921 Census, where the name on the record is Priscilla Rees.
My preference is to attach the record with the name as shown on the record first. For the 1911 Census, I might attach the record in the ordinary fashion, which links the associated source to the name (and records the event as a Residence). If I am creating a custom Census event, there may not be a source to associate with this event, but I can write down where the name came from in the person's Notes (private) or leave a Comment (public). If I have seen the record on findmypast, I often copy the address and the transcription of a person's entry into the description field for the event, so the name appearing on the record is shown on the profile.
However, it is usually suggested that we record women by their maiden names. Suppose I don't know the maiden name yet. In that case, my next step is to add a name "Priscilla" with no surname and mark that as the preferred name. Some users dislike leaving the surname field entirely blank. Crista Cowan has stated in one of her "Barefoot Genealogy" videos (on the Ancestry Desktop Education Playlist on Ancestry's YouTube Channel) that entering five underscores to indicate the field is blank on purpose will not interfere with Ancestry's search and hint algorithms. Using abbreviations such as UNK for unknown or LNU (last name unknown) is not good practice (see the Further Reading section).
Further along in the resarch process, I might acquire the birth certificate from the GRO, and add the name Fanny Priscilla Selby. Ancestry has a feature where we can add sources for items that aren't found on Ancestry, but I don't use it for reasons that aren't relevant to this answer; my full citations are in my desktop software, Family Historian. Instead, I make a note in the description of the corresponding event to remind myself where that form of the name was seen. I could also use the Notes or Comments. The advantage of putting it in a description field is that the information is visible and preserved if I print the profile to a PDF.
If the information comes from a baptism record, I usually put a mini-transcription of the register in the description field, "Fanny Priscilla daughter of [parents] Selby" with a note like "fmp image" to note that I've seen it.
Finally, suppose I knew from personal knowledge (either my own, or people who knew her personally) that Priscilla hated the name "Fanny" and always went by Priscilla. In this case, I would enter the name Priscilla Selby and mark it as the Preferred name, again leaving a Note or Comment (or both) to record how I knew that was the case.
Do what works for you. Whatever you choose to do, do keep a record somewhere with a list of the variant names you've seen, noting which sources they came from. One of the downsides of the online Ancestry tree system is that it is very easy to over-write the preferred name when attaching records, and thus getting the associations of which source has which names muddled. If you keep a separate list in Notes, you can go back and fix things when a merge clobbers your prefered name.
Further Reading
From Tamura Jones' Modern Software Experience:
From Ancestry Support:
