Add Google Analytics _ga parameter when pointing from own domain to shopify domain

RNL
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Goal:
Currently traffic from domain-a.com towards domain-b.com is seen as new traffic because Google Analytics does not see the user as 1 single user and thus creates a new client ID and new session. Result of this is that on domain-b.com we have 'domain-a.com' as traffic source.

Impact:
Traffic we buy (for domain-a.com) can not be attributed to the sales we make at the shopify domain.

I have:
1) my own domain (domain-a.com)
2) a shopifyfy domain (domain-b.shopify.com) 

What we did:
a) On domain-a.com i added a buy-button that opens a panel (basket) on the right with the selected line items. If i click 'order' it goes to domain-2.com all correctly
b) on both domains, the same GA tracking property is available
c) the correct allow linker settings are in place (for example on domain 1 i put --> ga('linker:autoLink', ['domain-b.shopify.com'], false, true );
d) the referral exclusion list is updated

Situation
Now what should happen when clicking on the 'order' button (in the right panel) is that the linker parameter is added to the url (it's _ga=somestring). But that parameter is not added. Info:
https://developers.google.com/analytics/devguides/collection/gtagjs/cross-domain

I checked that the panel is actual in some kind of iframe. But when i do view source it's empty. The iframe tag also does not have a 'src' element.
<iframe horizontalscrolling="no" verticalscrolling="no" allowtransparency="true" frameborder="0" scrolling="no" name="frame-cart" style="width: 100%; overflow: hidden; border: none;"></iframe>

Question:
How can i implment that when the user clicks 'order' in the panel, the user is redirected to domain-b.shopify.com with the _ga parameter as querystring?

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