# POST /opportunities/:opportunity_id/billing-parties/:customer_id

**Resource:** [Opportunities](./opportunities.md)  
**MCP tool:** `add_opportunity_billing_party`  
**Scopes:** `opportunities:write`, `invoices:write`  
**Write operation:** yes

Add a billing party (payer company) to an opportunity. Invoices raised in your accounting integration to that company then surface on this opportunity in addition to the client’s, without changing the opportunity’s client. Use when a related entity pays the fees for the matter. A manually pinned invoice (linked directly to another opportunity) is never pulled in.

## Parameters

| Name | In | Type | Required | Description |
|------|----|------|----------|-------------|
| `opportunity_id` | path | string | yes |  |
| `customer_id` | path | string | yes | The crm_customers id of the paying company. |
| `notes` | body | string | no | Optional note, e.g. why this entity pays the fees. |
| `require_approval` | body | boolean | no | Optional. Set true to route this write into the approval queue for human review instead of executing it immediately (returns 202 + an approval_id). Works even when your key/token has permission to execute directly. |

## Request example

```bash
curl -X POST   "https://api.trustpager.com/functions/v1/api/v1/opportunities/:opportunity_id/billing-parties/:customer_id"   -H "Authorization: Bearer YOUR_API_KEY"   -H "Content-Type: application/json"   -d '{"notes":"..."}'
```

---
Base URL: `https://api.trustpager.com/functions/v1/api/v1` — Auth: `Authorization: Bearer YOUR_API_KEY`