I am attempting to create a custom job number/invoice number for each entry into my worksheet.

An example goes as follows:
Organisations in column A, e.g. 'Google', Prefix based on Organisation column B, e.g. 'GOO'. I then have a number that follows this. The number is associated to a specific organisation in that each organisation starts at 1 and increases thereafter with repeat entries. e.g. GOO1, GOO2, GOO3, etc.

I don't know how to construct this. Any tips?