A number starting with MP is the readable reference every demand gets — for example MP-2026-000042. You use it instead of the demand's technical id: it is easy to say, easy to type and easy to paste into a search box.
The number appears wherever the demand itself appears: on the demand card in Marketplace, in the heading of the demand's detail page, on each row under My Demands, in the demand summary on the Make an Offer screen, and in your My Marketplace Offers detail. The search box on Marketplace accepts it too — the box reads "Search: title, category, or demand no. (MP-…)". Paste the number and you land on the demand in one step.
When you flag something with Report, the demand is identified by this number as well, so giving support the number is enough — you don't need to send anything else.
You don't choose the number: it is assigned automatically when the demand is created and never changes afterwards. Demands opened before this was introduced may not have one; in that case the field simply stays empty rather than showing broken text.
Also asked as: what does the MP number mean? · what is the demand no.? · where do I find my demand's reference number? · can I search by demand number?