Hi - the script step "Show Custom Dialog" will give you the option to ask the user a question and allow them to enter a value into a field. When you are setting up the script in the Show Custom Dialog Options select the Input Fields to specify what field you are wanting the report number to saved in. You will then need to have your script look at the value entered in the field and run whatever report is required based on the value entered.
The options shown above result in a custom dialog as below.
Hope this helps. Chris