drop down links to specific customer

I was hoping that someone might be able to help me out with an issue I'm facing. I have multiple customers that are located in my campaign monitor admin panel. In the form on my site I have fields for first name, last name, address, email address, and MOST IMPORTANT a drop down with all of my customer names. I would really like to be able to choose a customer from the drop down and the form field info would be sent to that customers client list. If someone could assist me in achieving this with PHP I would be most grateful. I'm completely stumped and this is actually way over my head (API thing). If this is something that needs to be done by a professional could you please give me a rough cost to do this.

Thanks in advance

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