Hi Jozef,
Thank you for contacting us!
Because DockingLayout component uses the Window Component we can use its API to select the tab you want.
Quick example on how to do this:

window.onload = function () {
    const docking = document.querySelector('smart-docking-layout');
    docking.layout = [
            //The Base Group
            type: 'LayoutGroup',
            orientation: 'horizontal',
            items: [
                    //DockingLayout Item B
                    label: 'TabsWindow B',
                    size: '25%',
                    items: [{
                            //Tab Item B1 of Item B
                            label: 'Tab B1',
                            selected: true,
                            id: 'clickEvent12321312',
                            content: 'Content of B1'
                            //Tab Item B2 of Item B
                            label: 'Tab B2',
                            content: 'Content of B2',
                        id: 1,
                        tabCloseButtons: true
    setTimeout(() => {
    }, 1000);

More info about this you can find in the window API here https://www.htmlelements.com/docs/window-api/
Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team