Pass the members of a datasource in to a query prompt

We need to pass the members of a dimension from Datasource 1 in to the prompt of Datasource 2

We have so far implemented:

var getmemberArray=DS_1.getMembers("OBJ_40",20);

var referenceString="";

getmemberArray.forEach(function(element, index) {

referenceString = referenceString + " " +element+";";

}

);

On the bolded, underlined row we get an error saying "Cannot concatenate objects of type"String" and "Member""

Anyone got any ideas?

I doubt it is this difficult to get the members of a DS in to a semi-colon separated string.