<script src="https://code.jquery.com/jquery-1.12.4.min.js"></script>
<script type="text/javascript">
var listName="CL0206";    
function AddListItem(){
    var itemType = GetItemTypeForListName(listName);
    var serverDetails='{"Server Name": "Production","Domain": "Gmail","Environment":"UAT"}';
    var item = {
        "__metadata": { "type": itemType },
        "Title": "testing",
        "ServerDetails":serverDetails
    };   
    $.ajax({
        url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listName + "')/items",
        type: "POST",
        contentType: "application/json;odata=verbose",
        data: JSON.stringify(item),
        headers: {
            "Accept": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val()
        },
        success: function (data) {
            alert('Success');            
        },
        error: function (data) {
             alert(JSON.stringify(error));
        }
    });
}
// Get List Item Type metadata
function GetItemTypeForListName(name) {
    return "SP.Data." + name.charAt(0).toUpperCase() + name.split(" ").join("").slice(1) + "ListItem";
}  
</script>
<input type="button" onclick="AddListItem()" value="AddListItem"/>