Answer
How do I request an APC waiver?
First check whether you need to. Wiley, Sage, Elsevier, Cambridge and PLOS apply reductions automatically from your affiliation. Taylor & Francis, Springer Nature, BMJ, BMC, MDPI and PeerJ require a request, and all of them require it at or before submission. State your institution and country; where a country list exists, that is the entire case.
Detail
What not to write
Personal circumstances do not strengthen a country-based request, because the decision is administrative. Where a discretionary route is being used instead, ACM states plainly that simply saying you cannot pay is typically insufficient.
Keep the confirmation
Invoices are occasionally issued in error after acceptance. A saved approval resolves it in one reply.