Cannot find the post method when doing a httpcallout through flows to convert an opportunity to salesagreement

In Salesforce, I am trying to convert an opportunity to sales agreement through HTTPCallOut Action using flows and it does not give me an option to select the POST method, it only has the GET method.

I created a legacy Named credential with the URL - https://xxx-c1.my.salesforce.com/services/data/v56.0/connect/manufacturing/sales-agreements.

Any inputs in this regard would be appreciated.

1. The manufacturing cloud developer guide says POST method is supported - https://developer.salesforce.com/docs/atlas.en-us.mfg_api_devguide.meta/mfg_api_devguide/connect_resources_sales_agreement.htm

2. I tried to edit the external service created by the flow with the httpcallout action and it doesn't allow to edit