const fetch = require('node-fetch');
const base64 = require('base-64');
const authUser = 'localbt';
const authPass = '<password here>';
const authString = 'Basic ' + base64.encode(authUser + ':' + authPass);
const btAPIBase = 'http://localhost:5159/api/actions';
const btAPIOptions = {
    Wait: '30s',
    MessageCount: '200',
    MessageSeverity: 'Info'
};
const btAPI = btAPIBase + '?' + Object.keys(btAPIOptions).map(key => key + '=' + btAPIOptions[key]).join('&');const btPrinter = 'wilkins-zebra2';
const btwFile = 'C:/label_files/gs1-128a.btw';
const actionScript = JSON.stringify({
    ActionGroup: {
        Actions: [
            {
                PrintBTWAction: {
                    Document: btwFile,
                    Printer: btPrinter,
                    SaveAfterPrint: false,
                    NamedDataSources: {
                        ship_addressee: 'ABC Company',
                        ship_address1: '1 Main St.'
                    }
                }
            },
            {
                PrintBTWAction: {
                    Document: btwFile,
                    Printer: btPrinter,
                    SaveAfterPrint: false,
                    NamedDataSources: {
                        ship_addressee: 'XYZ Company',
                        ship_address1: '1 High St.'
                    }
                }
            }
        ]
    }
}, null, 4);

console.log('sending', actionScript);
sendByFetch();
function sendByFetch() {
    url = btAPI,
    fetch(url,
        {
            method: 'POST',
            headers: {
                'Authorization': authString,
                'Content-Type': 'application/json'
            },
            body: actionScript,
            credentials: 'include'
        })
        .then(function (response) {
            var contentType = response.headers.get('content-type');
            if (contentType && contentType.includes('application/json')) {
                return response.json();
            }
            throw new TypeError('Oops, we haven\'t got JSON!');
        })
        .then(function (json) {
            console.log('response: ',json);
        })
        .catch(function (error) {
            console.log(error);
        });
}

I have a couple of questions, but to make this less messy I'll ask them as replies to this post, if I can do that.