How to Create a Record Using Web API

// define the data to create new account
var data =
    {
        "name": "Sample Account",
        "creditonhold": false,
        "address1_latitude": 47.639583,
        "description": "This is the description of the sample account",
        "revenue": 5000000,
        "accountcategorycode": 1
    }

// create account record
Xrm.WebApi.createRecord("account", data).then(
    function success(result) {
        console.log("Account created with ID: " + result.id);
        // perform operations on record creation
    },
    function (error) {
        console.log(error.message);
        // handle error conditions
    }
);

Source: createRecord (Client API reference)