How to fetch dynamic value in 3rd Party Tags in DTM ?

Hi All,

We have some code snippet which we want to place on the page and I can use 'Javascript/Third Party Tags' to do that. 

But the challenge is, we have some fields in the snippet which requires data to be picked up from the page dynamically. Please see below one sample code snippet:

<script>
demo.identify({
customer_id:'11111', // TODO: Replace with your customer identifier
email: 'johndoe@somedomain.com', // TODO: Replace with your customer's email address
joined_at: '2014-08-24’, // TODO: Replace with your customer's join date
firstname: 'John', // TODO: Replace with your customer's firstname if available
lastname: 'Doe' // TODO: Replace with your customer's lastname if available
});
</script> 

These all fields needs to be populated from the page dynamically. The page has these details available. 

Please suggest, how can the data be picked from and then how these picked data be fetched here in the snippet ?

Thanks,

Adwait