You can't
Experience builder basemap widget don't have create a new basemap option.
Not like web appbuilder, you can
https://transparentgov.net/cleargov1/1420/how-to-add-nearmap-image-server-as-base-map-on-web-app-builder?show=1420#q1420
So what is work around?
You can add nearmap image server layer as basemap in web map.
Step 1
find the nearmap portal item, if you don't have yet, create a new portal item
Step 2
Step 3
open web map
Step 4
paste URL here, don't forget to check USE AS Basemap option.
Step 5