Add allowLinker to supported field names

Not really. It's an cross domain tracking key only and the module does all the magic for you - automatically! Why are you trying to add it manually?

What do you mean "the module does all the magic for you - automatically"? In old Analytics we had to use these lines for cross-domain tracking:

_gaq.push(['_setDomainName', '.example.com']);
_gaq.push(['_setAllowLinker', true]);

The recommended way to do cross-domain tracking now is by doing this is it not?

ga('create', 'UA-XXX-Y', 'example.com', {allowLinker: true});

Bullshit. In both previous and today versions the module adds this automatically for you if you configure multidomain. If you do it yourself you run into a lot of fail functions.

So that multi-domain setting forces you to enter all the domains we want to track. We don't even know all the domains that might have outbound links on a website so we need to be able to check multi domain but NOT specify the domains. If there is a way to do it just let me know and we'll try it.

Google does not support that you do not specify Your cross domains. You must specify Your cross domains or it cannot work.

Google actually does support this and a whole lot of other "Create only fields" options now: https://developers.google.com/analytics/devguides/collection/analyticsjs.... The patch at the top of the page works perfectly.