To access Sweet for ArcGIS Online, the administrator of your ArcGIS Online account will first need to assign a Sweet licence to your ArcGIS Online user. Please see the Licensing page for further details.
Once this has been done, you will be able to access the Sweet for ArcGIS Online Builder through the App Launcher.
Alternatively, you can use the URL https://www.sweet.esriuk.com to access the Sweet for ArcGIS Online Builder app. Click the Sign In button and use your ArcGIS Online credentials to access Sweet (or, if you are already logged in with your current browser session, click Allow to give permission to access you ArcGIS Online account information).