Just checked client original site. 
No GTM, Google Analytics or any G tag on site. 

Let's make one- are we doing a micro site or building on the current website Chris  ?

Got it, done. 
GTM account created for client. 
Added other team members to ADMIN, sent invites. 
Added Optinmonster container tag.
Updated client's spreadsheet.

<!-- Google Tag Manager -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-WDH662F');</script>
<!-- End Google Tag Manager -->

<!-- Google Tag Manager (noscript) -->
<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-WDH662F"
height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
<!-- End Google Tag Manager (noscript) -->