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();