This is accurate. You can do something like the following.

javascript
cy.fixture("calculator/monthlyRepayment.json").then((json) => {
  cy.intercept("POST", "**/full", (req) => {
    //you can use the request here if you need data that is included in the request
    let name = req.body.username;
    json.username = name;
    req.reply(json);
  });
});

You can see more documentation here: https://docs.cypress.io/api/commands/intercept#Controlling-the-outbound-request-with-req-continue