Go to Home tab, paragraph section and click on the down arrow next to the numbering button. Choose define new number format. First you can choose numbering style like numbers or letters. In Number format you can add custom text to your numbering. For example we are going to add text before numbers. So now all the list numbers will automaticly have text written before them. If you want to use that numbering again in the document, it is now stored in the Numbering Library