Create a Custom Index Column in Power Query

1. Open Power Query Editor: Click on the Add Column Tab, Index Column > From 1

Then from Add a Custom Column as well:  click on Custom Column.

Enter Custom Formula: In the Custom Column dialog box, enter the following formula:

This will pad your index numbers with leading zeros, resulting in 5-digit codes.

Once created delete the original Index Column.

If you would like to add a Prefix as well to the custom Column, then:

Click on the TRANSFORM tab > Text Column > Format > Add Prefix