You have a few logs of types that are logged in Log Analytics by Application Gateways. Let's assume you are talking about access logs.
Example query for that would be
AzureDiagnostics
| where Category == "ApplicationGatewayAccessLog"
| where requestUri_s contains "myadmin"
you can replace 'myadmin" with something else. Here is the documentation on contains and the other string operators:
https://docs.loganalytics.io/docs/Learn/Tutorials/String-operations