Bill.com API Works in Postman, but not Retool

I want to make an app that uses my billing software's API to show who our vendors are, create new vendors, and invite vendors to the software's network. I am new to Retool, so I don't know all the capabilities and what not. After looking in Retool's documentation I still can't figure out what could be causing the issues.

So far, I have been successful in all my API requests in Postman. However, when I transfer those same requests to Retool I get error messages. I tried changing the body to JSON and the same error message comes up. I don't know if I am missing something, but nothing seems to work. At the moment, the only request that is working is the login request.

Your request has an extra , in the request.

Your request
{"start":0,"max":999,}

Correct request
{"start":0,"max":999}

I fixed it and now it has a new error message: "Session is invalid. Please log in." I am copy and pasting the sessionId straight from the response. Is there a way I can make a global variable that I can call in the body for sessionId?

Session invalid means your auth token has expired.

You can store the token locally or in a variable while testing

So if I wanted to make sessionId a variable that I can call in another query, but the actual value of the variable changes each time I login. I would put the code in the Success Handler? What it be? I tried:
const sessionId = loginQueryv2.data.response_data.sessionId;

if (sessionId) {
await sessionId.setIn(sessionId);
}

That didn't work though.