You can ask questions to the customer using the public questions and answers feature. To submit a question to the customer, open the project details and scroll down to the "Questions from Pros" section. Click the "Ask Question" button, ask your question in the provided text box, upload any related files when applicable, and click "Submit Question." Your question will be sent to the customer and you'll be notified when they've answered your question. It's important to note that your question and customer's answer will be made publicly visible for other pros to use when putting their own bids together.
Alternatively, if the customer has already started a chat with you or they have enabled messaging them, then you can send your questions privately to the customer using the chat page.