Restapi to update choice field with new vvalues dynamically

I am trying to append dropdown dynamically from textbox.. I am trying to append hardcoded value to choice but I am getting error.. I neeed to append new value. choice fied has values already. please help. its rugnet

$.ajax({

url: endpointUrl + "/_api/web/lists/GetByTitle('Pages')/fields/GetByTitle('CallAction')",

type: "POST",

data: JSON.stringify({

"__metadata": { "type": "SP.Data.PagesItem" },

"CallAction": { "__metadata": { 'type': "SP.FieldChoice" }, "Choices": {results: ["Test1"] }},

}),

headers: {

"Accept": "application/json;odata=verbose",

"content-type": "application/json;odata=verbose",

"X-RequestDigest": $("#__REQUESTDIGEST").val(),

"IF-MATCH": "*",

"X-Http-Method": "PATCH",

},

success: function (data) {

alert("Success");

},

error: function (err) {

alert(JSON.stringify(err));

},

});