Answer this question with more than just a one-word (Indeed, Google, etc.) answer. You want to let them know where you found the job, but also why you were looking for this type of job. Talk about how long had you been looking for a product manager job and why this position sounded like the right one for you. If you know someone who works there who recommended it to you, make sure to mention them by name. This is an opportunity for you to point out why they want to hire you.

"I came across your job posting and applied for it through Indeed. I had been looking for an opportunity with an innovative company for the past several months, and while I had come across several product manager postings, none of them seemed like the right fit for me. My cousin's wife works in the Human Resources department at your company and she's always talked about how much she enjoys being a part of your organization, so when I saw the posting for this position, I immediately called her to ask some questions. She told me about the family-focused culture and fun atmosphere in the office, and I already knew about the innovative products you bring to the market. After talking with her, I knew this was an amazing opportunity to join a great company and I completed the application online."