The apartment was available through the website’s “instant book” feature, which allows anyone with an Airbnb account to book a stay without so much as a message to the host. A New York Times reporter reserved it this way last month for a weekend stay in April.

“Welcome!! Looking forward to meeting you!” the host, Lena Yelagina, wrote back.

She said she would meet the guest downstairs and show him around. “Can you please do not tell building staff that it’s Airbnb but that you are rather visiting me,” she wrote. “I will really appreciate it!”

But two days later, Ms. Yelagina wrote again to say that she had discovered that the guest was a journalist and that she did not want her apartment to be used to learn anything about Mr. Trump or to be featured in an article.

“I apologize for this request but I have to make sure that we have a precise agreement and will not have any problems,” she wrote.

After the reporter informed her that he planned to write an article, she canceled the reservation and did not answer any questions. Public records indicate that Ms. Yelagina has owned the apartment since 1998 and is listed as an owner of a condominium on the Upper West Side.

How the listing was able to float under the radar in such a high-profile building remains a mystery. It is illegal under state law to advertise and rent most apartments in New York for fewer than 30 days when the host is not present. The Trump Tower listing advertised the entire apartment and said that it could be rented for as few as three nights.