Click the button Add new WMS source

In the dialog box, enter WMS URL (GetCapabilities) and then press the Read button.

If you want to try it out, you can use this demo:
https://demo.boundlessgeo.com/geoserver/ows?request=GetCapabilities&service=WMS

If WMS URL you entered is correct, then a list of available layers will be displayed. Select the ones you want to share with users of the Mapit gadget, fill in the Attribution field and then press the Add new WMS source button

As a result, you will receive a new row in the table (as shown below). Enter the name that will be visible in Mapit gadget and check the checkbox in the Show column - to share the WMS layers.
You can also make this the default layer, by choosing the Default radio button.

The Details button will allow you to return to the dialog box with previously selected layers, to makecanges .
You can delete the data row with the Delete button, as long as it is not set as the default map source.

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