If you cannot select a listed service provider, confirm that a Data Destination was added via Integrations - Data Destinations. If a Data Destination is absent, select Add Destination and choose a preconfigured monitoring platform to receive your data.
If a Data Destination is visible, click on the Destination and check "Allow forwarding of all my Custom metrics to this new data destination," Save Changes." If this box is unchecked, you cannot create custom metrics and forward them to a service provider.
After saving the changes, return to the Custom Metrics tab, Add a Custom Metric, Define the query and output.