How to use the azure-devops-node-api/WebApi.getHandlerFromToken function in azure-devops-node-api

var definitionIdTriggered: string = null;
        var buildId: number = null;
        var buildVersionToDownload: string = tl.getInput("buildVersionToDownload", false);
        var allowPartiallySucceededBuilds: boolean = tl.getBoolInput("allowPartiallySucceededBuilds", false);
        var branchName: string = tl.getInput("branchName", false);;
        var downloadPath: string = tl.getInput("downloadPath", true);
        var downloadType: string = tl.getInput("downloadType", true);
        var tagFiltersInput: string = tl.getInput("tags", false);
        var tagFilters = [];
        if (!!tagFiltersInput) {
            tagFilters = tagFiltersInput.split(",");
        }

        var endpointUrl: string = tl.getVariable("System.TeamFoundationCollectionUri");
        var accessToken: string = tl.getEndpointAuthorizationParameter('SYSTEMVSSCONNECTION', 'AccessToken', false);
        var credentialHandler: IRequestHandler = getHandlerFromToken(accessToken);
        var webApi: WebApi = new WebApi(endpointUrl, credentialHandler);
        var retryLimit = parseInt(tl.getVariable("VSTS_HTTP_RETRY")) ? parseInt(tl.getVariable("VSTS_HTTP_RETRY")) : 4;

        var templatePath: string = path.join(__dirname, 'vsts.handlebars.txt');
        var buildApi: IBuildApi = await webApi.getBuildApi();
        var artifacts = [];

        if (isCurrentBuild) {
            projectId = tl.getVariable("System.TeamProjectId");
            definitionId = '';
            buildId = parseInt(tl.getVariable("Build.BuildId"));
        }
        else {
            var releaseAlias: string = tl.getVariable("release.triggeringartifact.alias");
            var triggeringBuildFound: boolean = false;
            definitionIdSpecified = tl.getInput("definition", true);