You can find the RequestId in every API answer even when you are not authorized yet. It’s the last info in the bottom of the answer. There's two ways to get it through the Authorize endpoint.
1. With Credentials
You have to enter the institution, the username and the password.
2. With LoginId
If you want to connect an account that was previously connected once, you can use its LoginId.
If you don't know where to find a LoginId from an account, click here.
The RequestId is good for a single workflow: Authorize -> GetAccountsSummary -> GetAccountsDetail/GetStatements. Every new workflow will have a new RequestId.