Create a script that will Email Subject from Text Field

Hello,

I am new to scripting.  Please bare with me.....  i've got my form all set. 

I am looking to have the user click on the "send email" button I created.  Then it will take the data the user entered in a text field (last name) of the form and put that into the subject line of the email before they send it.

So the suject line should read something like:  New User - Williams

i have scoured the forums and found some help but not in full.  I found the scripting section but don't know how to create it or what the script should look like.

Any help would be greatly appreciated.

Thank you for your patience!