What version are you using, FMP 10? If so, you can set up a script trigger to do this for you. There are also other approaches by using calculation fields and or merge fields on your layout that can create the same effect.
Here's an approach that works with any version of Filemaker:
Define two text fields. For my example, I'll call them "Greeting" and "Customer Name" and make them part of a table called "Letter". (You'll need to use the field and table name from your own file in place of these.)
Define a calculation field named "Salutation" and give it the following expression: Letters::Greeting & " " & Letters::Customer Name. Set it to return text.
Now place These three fields on a layout.
Format Greeting as your pop up menu.
When you select a greeting and type in a customer name, Letters::Salutation will display the text you need for the start of your letter.
As I said at the start, there are other approaches. If this method does do what you need, post back with more info and we'll take it from there.
Thank you very much
It works a treat!