You can add the property to web property bag from designer.
Below is the code that you can use to get the added property
ExecuteOrDelayUntilScriptLoaded(getWebProperties, "sp.js");
var webProperties;
function getWebProperties() {
var clientContext = new SP.ClientContext.get_current();
webProperties = clientContext.get_web().get_allProperties();
clientContext.load(webProperties);
clientContext.executeQueryAsync(Function.createDelegate(this, this.getWebPropertiesSucceeded), Function.createDelegate(this, this.onQueryFailed));
}
function getWebPropertiesSucceeded() {
//debugger; //use this to force a break here
//returns an object with all properties.
//Use the quick watch to expand this out to see all of them.
var allProps = webProperties.get_fieldValues();
var customProp = "";
//make sure the property is there before using it.
if(webProperties.get_fieldValues().YammerAccessToken != undefined)
{
var customProp = webProperties.get_fieldValues().YammerAccessToken;
}
alert(customProp);
}
function onQueryFailed(args, sender)
{
//handle errors here
alert("Error : " + args.get_message());
}
getWebProperties();