To set up automatic cross-domain tracking with gtag, you'll need to add Maker to your referral exclusion list and automatically link your primary domain with Maker's secondary domain.
Note: You'll need a developer's help with the second part.
Add Maker to Your Referral Exclusion List
To ensure that Google Analytics treats traffic from Maker as direct traffic instead of referral, you will need to update your referral exclusion list.
Open the Admin Panel in Google Analytics, then select the Tracking Info menu item from the PROPERTY configurations (ensure the correct site is selected in the ACCOUNT settings as well) and select Referral Exclusions List.
To add Maker as a domain, click +ADD REFERRAL EXCLUSION.
Enter Maker's Domain name: App.maker.co and click Create to save:
Configuring a site to accept linker parameters (developer task)
Once a user arrives at a page on the destination domain with a linker parameter in the URL, gtag.js needs to know to look for that parameter.
If the destination domain has been configured to automatically link domains, it will accept linker parameters by default.
If the destination domain is not configured to automatically link domains, you can instruct the destination page to look for linker parameters by setting the accept_incoming property of the linker parameter to true on the destination property's config: