Add a field to the Umbraco 8 Members Index

I have extended the Umbraco 8 Member type by adding some custom fields.

I need to be able to search Members by one of these fields, 'organisationName' (that's it's alias), so when looking at Examine, I tried to add this to the members index like this:

private readonly IExamineManager _examineManager;

    public CustomizeIndexComponent(IExamineManager examineManager)
    {
        _examineManager = examineManager;
    }

    public void Initialize()
    {
        // get the external index
        if (!_examineManager.TryGetIndex("MembersIndex", out var index))
            return;

        // add a custom field type
        index.FieldDefinitionCollection.TryAdd(new FieldDefinition("organisationName", FieldDefinitionTypes.FullText));

    }

when I put a breakpoint on after the TryAdd I can see the new field, but in the back office it's not there when I look at the members index.

Am I going about this the right way, as in can I actually add my field to the members index, or should I create a new custom index based on the member?

I think most people create their own index like here: https://our.umbraco.com/forum/developers/extending-umbraco/72299-umbraco-7-backoffice-member-search-by-custom-properties#comment-277928

But I would personally just access the membership API with GetMembersByPropertyValue. It's very easy to make a call to the member api with umbraco api controller. https://our.umbraco.com/documentation/reference/management/services/memberservice/ (here an example just to show the few lines).

/umbraco/api/SearchMemberApi/ReturnMembersWith