First Verify what cores are available in the Solr app.
Did you go through the process of setting up the cores for Sitecore in the config files?
The "Populate Solr Managed Schema" should contain the cores you have defined for the indexes in Sitecore, then push those cores to the Solr server.
Sitecore documentation here
If all that is good,assuming you have VNets (recommended practice) for both your Web apps,next you'll want to check the communication between the two VNets of the Web Apps,i.e setup VNet peering and allow IP access rules between the Solr VNet and Sitecore VNet.
Azure docs here
-If no VNets are setup then just add the address of your Solr web app to the config file.
Set up the base address for solr in the "Sitecore.ContentSearch.Solr.DefaultIndexConfiguration.config" to point to your Solr web app IP.