How can I add a new client's name that isn't listed in the list under "Client's Name"?

Trying to add a new client under the "Client's Name" section but will not let me because you must select a client that is already listed below.

It is only possible to select from options currently in a drop down field.  If you need more options, it would require editing the form to include the additional options.  If it is one of your own forms and need help with how to update it, let us know which form you are editing and we can take a look.