Here’s the recommendation I got from Galaxy Support (the vendor of the host I want to create a Zap for; note their API documentation):

...I am pretty familiar with how API authentication works in general, which is why I was able to help troubleshoot the last step in more detail. However, this next piece isn't something I have worked on before, so unfortunately I am not able to advise on how to set this up in Zapier. This is the part where I would recommend reaching back out to Zapier for help, and I would be glad to advise on what questions you should ask them. 

Let's use the /users/list endpoint as an example. This endpoint will show you a list of users on the site, and is a good one to use for testing because it's straightforward and easy to verify the results. 

If you were to make a call to this endpoint in Postman or some other API client outside of Zapier, the actual request would look something like this:

In this example, I am using curl, which is a command line interface for making API requests. The reason I'm using this format is that it's very common, and Zapier should be able to help translate this request into what you need to enter into their actual web interface:
curl --location 'https://api.galaxydigital.com/api/users' \

--header 'Accept: application/json' \

--header 'Authorization: Bearer

(long key deleted)'

I would go to Zapier with this request, and ask them how you need to set up your configuration to work in their platform. They will also be able to help you troubleshoot any errors you're getting much more effectively! 

I'm glad to advise where I can here, but unfortunately, there are some things that are outside my wheelhouse that I don't feel confident in providing instructions for. Let me know if you have any questions about what you need to ask Zapier, or if you have any follow-up questions for me after you talk to them!