Currently the only way to create and or edit the Format String Expression is in Tabular Editor. Whilst I love Tabular Editor it does not have any DAX Intellisense.
I kept on editing the Format String Expression and I honestly must have had to edit it 4 times and it still was wrong.
Below is a quick tip on what I did to make sure my Format String Expression was valid and would indeed work!
Read on to see how.