Paige,
Are you talking about one of the online app templates, Web AppBuilder, or what ArcGIS Online web application specifically?
I am looking to use one of the configurable app templates. For example starting with the Story Map Journal Configurable Application.
Paige,
OK the all the configurable app templates are developed with the JS API and thus you can interact with other APIs using the JS APIs esriRequest class to make a request to the third party API and get you results and then work with them in JS code to add them to the map.
So I need JS on my computer to work with the API data and add it into the web map? I am a beginner with using JS and API.
Paige,
You would need to download the app template and then choose a JS IDE (I use Adobe Brackets) and then have some level of JS programming in order to modify the app js file(s) to add the custom code to use other APIs (probably not something a beginner in JS is going to be able to do).