Autofilling text special character if new cell/row is empty

[jmccall93]

I have an invoice spreadsheet and instead of being blank I would like the spreadsheet to enter # in each new first cell of a newly added I'm thinking a IS BLANK row, but leave my invoice number fine if I end up populating an invoice number behind the #. I'm thinking an ISBLANK formula but I cant figure out how to specify that the False portion needs to be changeable for random invoice numbers filled in.

 

[SunnyAlv]

Can you give an example of a picture or table to make it easier for someone to answer your question?

 

[jmccall93]

Here is a sample of my spreadsheet. Once you open it, in the first column every invoice is preceded with a # sign. Id like the pound sign to automatically appear and them allow me to add individual invoice numbers behind it.