1. When you open a request generated by SoapUI, at the bottom you will find two tabs named Auth and Headers. When you click on the Headers tab, it will open a small window which allows you to add any headers to the message. There you can add a header name "Authorization" and its values should be "Basic <Base64 encoded username:password>".
If your username and password are admin:admin, then when you encode it, your header values will look like "Basic YWRtaW46YWRtaW4="
After setting this, if you send a request in SoapUI, you will be able to see the Authorization header in the request.
2. Other option is to use the Auth tab and define the username and password there. But, these information will not get sent as a header. To send them as a header, you need to do the following.
Open the "Preferences" window by clicking the icon or from the SoapUI menu. Then select "HTTP settings". From that settings list check (tick) the "Authenticate Preemptively" option as shown in the image below.
Then, set the username and password in the Auth tab as I described previously and send a request. You will see the Authorization header sent with the request as shown below.