Help with Online Custom Forms Needed.

We are trying to associate an online form to capture data for a custom record but find that the Custom 'Customer' Field that we have is displayed as a drop-down box (exposing all of our customer names) and that the drop-down is truncated through results that begin with HTH. Does anyone have a suggestion as to how to make this custom field searchable versus a drop-down list? Also, having the ability to require the customer to be logged in prior to using the form would allow for the use of the dynamic default being the current user.  Thanks in advance