Fluent Forms WP Plugin - Create a unique number for each submission

Hi there,

I'd like to know how to generate a unique string that can be used as a reference number for each entry submission in Fluent Forms? I need something like ORDER0001 and make it consecutive (ORDER0002, etc). One for each entry submission.
Apologies if it's an easy question, I'm not an expert in Fluent Forms.
Many thanks!

Gaby