You can also simply edit the signin.html file found in the Portal installation directory:
[Portal Installation Folder]\ArcGIS\Portal\webapps\arcgis#home\signin.html
I added a <div> with a logo and some text right before <div id="signInStyle"></div> in the signin.html file. You could change the banner style within the signin.html file as well.
I also edited the classificationBanner property in Portal's config.js file (see Set advanced portal options—Portal for ArcGIS (10.6) | ArcGIS Enterprise)