A custom font can be registered with the engine so the dashboard renders and exports with it.

In React.
import { Designer, Stimulsoft } from "stimulsoft-dashboards-js-react/designer";

function App() {
    var report = new Stimulsoft.Report.StiReport();
    report.loadFile("Dashboards/CusomFontDashboard.mrt");

    // Adding Custom Font
    Stimulsoft.Base.StiFontCollection.addFontFile("Fonts/LongCang-Regular.ttf");

    var options = new Stimulsoft.Designer.StiDesignerOptions();
    options.appearance.fullScreenMode = true;

    return <Designer options={options} report={report} />;
}

How it works. The engine loads the font before rendering, so custom typography appears in the designer, viewer and exports.