It seems very odd that you don't want to pull up a customer name from an existing table nor save it if a new customer, but that's a business procedure issue--not a database issue.
You can use global fields for data that you do not intend to save. It's the closed equivalent to those unbound text boxes used in Access forms and reports--though sometimes you can also use a variable as merge text if you are using FileMaker 11 or later.
You can use Show Custom Dialog to open up a custom dialog with up to 3 input fields--which can refer to fields with global storage. The user can enter data into these input fields and then you put the global fields you specified as input fields where needed on your report. In a networked system, clients of the hosted database can enter data in a global field and other users will not see that data and the field will return to it's original value (empty if you put it up on the network with the field empty) when they close the file. Your script that opens the custom dialog can first clear these fields with Set field [Yourtable::globalField ; "" ] before opening the dialog so you don't see the value from the previous report if you produce more than one in a single session.
If show custom dialog is too primitive for what you need. It's possible to use New Window to open a small window to a specified layout containing your global fields. In this case, you can format the layout and the fields in ways not possible with show custom dialog, but it takes extra scripting to make the window "modal" and get it sized and positioned the way you want without messing up the original window (If you are using maximized windows on a windows platform).
Note that global fields are also a useful way to specify criteria used in a script to perform a find.