WSO2AM Mediator with conditional properties

Question

We are trying to add an API to WSO2AM which has the problem, that we need to add (invisible for users) an Authorization Key.

We are able to add it using Mediator with that configuration:

<sequence xmlns="http://ws.apache.org/ns/synapse" name="Add_Fahrplan_AuthKey">
    <property name="REST_URL_POSTFIX" 
                  expression="fn:concat(get-property('axis2','REST_URL_POSTFIX'), 
                                        '&amp;authKey=**HERE_IS_THE_SECRET**')"
                  scope="axis2"
                  type="STRING"/>
</sequence>

We have 4 get methods

/location
/departure
/arrival
/journeyDetails  <-- needs an different AuthKey

But for the fourth get method journeyDetails we need to add the AuthKey in a different way with different characters.

Is it possible to do that in a Mediator configuration? And in case of yes, how can we do it?

Please keep in mind that we are not very familiar with synapse configuration syntax. We tried

value="/location" 

but it resulted in an AM-error while invoking the API and we tried

<filter> with <then> and <else> 

which resulted again in an AM error.

Thanks for any help