For example, here is the Create button in the Develop stage of the AccountToContactProcess sample business process flow that lets you create a Contact record from the Account form.

Below snippet is from Microsoft doc which needs some correction:
https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/reference/formcontext-data-process/stage/getnavigationbehavior

function sampleFunction(executionContext) {
    var formContext = executionContext.getFormContext();
    formContext.data.process.getActiveStage().getNavigationBehavior().allowCreateNew = function () {
        if (formContext.data.process.getName() === 'Test Process') {
            return false; // Create button is not available
        }
        else {
            return true; // Create button is available
        }
    }
}