To access and display raster layers in ArcGIS Online from a web map service (WMS) endpoint, you'll need to obtain the correct URL. Here's how:

1. First, navigate to your project or organization's settings within Arc GIS Online.

2. In this section, locate the 'Layers' tab.

3. Underneath it is an option labeled 'Add Layer From Service'. Click on that.

This will open a new dialogue box where you can input various types of URLs for adding layers from different services. Since we're focusing specifically on WMS sources:

4. Select 'WMS (Web Map Services)' as the service type.

5. In the next section, there are several fields to fill in:

- Enter your WMS server URL into the first field labeled 'Server'.

This should be followed by a question mark (?).

- Next comes the layers field where you can input either an individual layer ID separated with commas (,) or use * for all available layers.

6. Below these fields are options to customize various aspects of how your layer is displayed, such as its transparency and whether it's visible upon adding.

Once you've entered in all necessary information, click 'Add Layer' at the bottom right corner of this dialogue box:

Your WMS layer should now be successfully added to ArcGIS Online.

Get wms url