Javascript Token based authentication for multiple domains.

We need a scalable solution to authenticate the user from our php web application and store the token with the client via the browser. We need other websites on other domains with our javascript to be able to read the token so the javascript can send it in a restful API call. Our problem is that we are storing the token in a cookie which has built in protection from other domains accessing cookie data (the token) on another domain. It is this security measure that we need to overcome or work around. Our current work around is to send the client with the token to each domain but this is not a scalable solution. The solution must be achieved with javascript and without access to headers or other server side mechanisms with exception being to our main site. We of course can make modifications there. Communication and attention to detail is critical in this project so please begin your proposal with the phrase "One giant leap" so that we know you have fully read and understood the requirements. Our best hope is that the solution might be achievable through the use of "JWT" and "localStorage". Project will be completed when we can use the provided scalable solution to pass data from domain to domain via javascript.