How to send a custom HTTP header to backend through WSO2 API Manager

When you publish a API using WSO2 API Manager it creates this kind of API configuration in built in service bus.

<api name="admin--Example_Rest_API" context="/exampleAPI" version="1.0.0" version-type="url">
        <resource methods="POST GET DELETE OPTIONS PUT" url-mapping="/*">
            <inSequence>
                <property name="POST_TO_URI" value="true" scope="axis2"/>
                <filter source="$ctx:AM_KEY_TYPE" regex="PRODUCTION">
                    <then>
                        <send>
                            <endpoint name="admin--Emojot_Rest_API_APIproductionEndpoint_0">
                                <http uri-template="http://localhost:3000">
                                    <timeout>
                                        <duration>30000</duration>
                                        <responseAction>fault</responseAction>
                                    </timeout>
                                    <suspendOnFailure>
                                        <errorCodes>-1</errorCodes>
                                        <initialDuration>0</initialDuration>
                                        <progressionFactor>1.0</progressionFactor>
                                        <maximumDuration>0</maximumDuration>
                                    </suspendOnFailure>
                                    <markForSuspension>
                                        <errorCodes>-1</errorCodes>
                                    </markForSuspension>
                                </http>
                            </endpoint>
                        </send>
                    </then>
                    <else>
                        <sequence key="_sandbox_key_error_"/>
                    </else>
                </filter>
            </inSequence>
            <outSequence>                
                <send/>
            </outSequence>
        </resource>
        <handlers>
            <handler class="org.wso2.carbon.apimgt.gateway.handlers.security.APIAuthenticationHandler"/>
            <handler class="org.wso2.carbon.apimgt.gateway.handlers.throttling.APIThrottleHandler">
                <property name="id" value="A"/>
                <property name="policyKey" value="gov:/apimgt/applicationdata/tiers.xml"/>
            </handler>
            <handler class="org.wso2.carbon.apimgt.usage.publisher.APIMgtUsageHandler"/>
            <handler class="org.wso2.carbon.apimgt.usage.publisher.APIMgtGoogleAnalyticsTrackingHandler"/>
            <handler class="org.wso2.carbon.apimgt.gateway.handlers.ext.APIManagerExtensionHandler"/>
        </handlers>
    </api>

When you want to send a custom HTTP header in the API request, you need to add this property to the out sequence of API. customHeader is the name of the header.

 <property name="Access-Control-Allow-Headers" value="customHeader,authorization,Access-Control-Allow-Origin,Content-Type" scope="transport"/>