Question is which serial number is requested ?
Is it s/n of remote controller or roller shutter and where I can find it ?
In the Velux KLF200 web software isn’t any serial number
By far the easiest method:
Go to your “Things” page and on the bottom of the screen there is an “Inbox”. Click it and OpenHab will show you Things that are available but not yet added to Openhab. You can sort them by binding to find the Velux discovered Things.
From here you can click the discovered Thing and add it (and it will show you the serial number as well).
In case there are no Velux items in there yet, once more go to your Things page and click the blue circle with the plus sign. Click the Velux binding and at the top of your screen you will have a blue “Scan” button. Click that button to discover your Velux devices and add them from there.
Usually you don’t have to change anything in the Bridge thing, except for… the password.
Standard password filled by openHab is “velux123”. There is another password on the backside of the bridge (beneath the SSID). Use that one for the bridge.
After that retry the inbox and the scan. The scan will reveal the serial numbers as well.
as far as I know not directly from the standard UI. The standard UI is as shown in your screenshot. In other UI’s (like HABPanel) you can use a slider item from 0-100. In rules you can give a command like itemName.sendCommand(50). Other UI’s most likely also support a slider type of control (I never had the urge to start with other UI’s).