Try it as below:
_spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('Site')/items?$select=Title,Keywords&$filter=substringof('test',Keywords)
If that doesnt work, you need to create a POST request as below:
function postJson(endpointUrl,payload,success,failure)
{
$.ajax({
type: "POST",
headers: {
"accept": "application/json;odata=verbose",
"content-type": "application/json;odata=verbose",
"X-RequestDigest": $("#__REQUESTDIGEST").val()
},
data: JSON.stringify(payload),
url: endpointUrl,
success: success,
failure: failure
});
}
function getListItems(listTitle, queryViewXml,success,failure)
{
var queryPayload = {
'query':{
'__metadata': { 'type': 'SP.CamlQuery' },
'ViewXml': queryViewXml
}
};
var endpointUrl = _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listTitle + "')/getitems";
postJson(endpointUrl,queryPayload,
function(data){
success(data.d.results);
},failure);
}
How to use it:
getListItems('Site',"<View><Query><Where><Contains><FieldRef Name='Keywords'/><Value Type='Note'>test</Value></Contains></Where></Query></View>",
function(items){
for(var i = 0; i < items.length; i++) {
console.log(items);
}
},
function(error){
console.log(JSON.stringify(error));
});
Reference - Using CAML with REST API