403 Access Denied on Tomcat 8 Manager App without prompting for user/password

This may be work.

Find the CATALINA_HOME/webapps/manager/META-INF/context.xml file and add the comment markers around the Valve.

<Context antiResourceLocking="false" privileged="true" >

<!--
<Valve className="org.apache.catalina.valves.RemoteAddrValve" allow="127\.\d+\.\d+\.\d+|::1|0:0:0:0:0:0:0:1" />
-->

</Context>

You can find more details at this page.