If you want to ask a question to a particular user, make sure to check how much they charge for their time and if they have specific services listed on their profile. This will provide you an indication of what this might cost you.
Once you are ready to proceed, simply connect with the other user and start the conversation so both of you can agree on the service you are looking for. The other user can then request for a payment to be made before the conversation continues. You have a chance to define precisely what you are looking for before paying for it.