Typical use case

You have collected all the data of the Payment Request (not including the identifier of the Buyer Bank), and you want to create a MyBank Payment Request.

Request body attributes

All attributes in the request body are mandatory except:

durationMillis (if not specified, a value corresponding to '15 minutes' will be set by default)
merchantTaxNumberClassification (only needed if the optional feature "SCT for fiscal benefits" is requested)
merchantTaxNumberID (only needed if the optional feature "SCT for fiscal benefits" is requested)
validityStartDateTime (if not specified, a value corresponding to 'now' will be set by default)
validityEndDateTime (if not specified, a value corresponding to 'now + 365 days' will be set by default)

About initiatingPartyReturnURL:
Your initiatingPartyReturnURL must include a placeholder "{transactionToken}": for example, https://www.mystore.com/transactionFromPaymentRequestHasBeenHandledByPayer?tt={transactionToken} would be a correct value.
If a MyBank Transaction has been started from this Payment Request, then this URL (with the placeholder "{transactionToken}" appropriately replaced with the "transaction token" uniquely identifying the MyBank Transaction) will be requested as soon as the final status of the Transaction is available. More precisely, this URL will be requested either because the Payer, after authorising or cancelling the Transaction, has asked to be redirected back, or because the transactionValidityEnd of the Transaction has been reached. In other terms, the fact that this URL is requested means that a Transaction was started from this Payment Request, and its final status is available, and ready to be retrieved by invoking transactionDetails.
When initiatingPartyReturnURL (with the placeholder "{transactionToken}" appropriately replaced) is requested you should typically:

Retrieve the final status of the MyBank Transaction by invoking transactionDetails.
Serve a confirmation page to the Payer, stating that the payment has been successful / unsuccessful.

Typical use of the response

You will store the paymentRequestID for later use: you may want to use it to invoke paymentRequestDisable or, after a certain amount of time, paymentRequestDetails.
The Payee will present the Payment Request to the Payer (typically, the paymentRequestID will be rendered as QR-code and displayed/printed somewhere).