Lindsay, in Portal 10.7.1 you'll need to edit some html script within the oauth2.js file. I'd be sure to make a copy of the original file before editing.
Found in:
[Portal Installation Folder]\ArcGIS\Portal\framework\webapps\arcgis#sharing\www\files\scripts\oauth2.js
Find the "content" div in the javascript file and add your custom html to the "font-size-1 page-title width-full" div. In my case I added a "float:right" logo just after this div as well.
<div class="content">\n
<div class="content-wrap">\n
<div class="js-header header-bar flex flex-justify">\n
<div class="font-size-1 page-title width-full">\n
City of Waco GIS Portal<br>
<small>
COW Employees may sign in with their employee credentials:
<br>Username: <b>COW Username</b>
<br>Password: <b>COW Password</b>
</small>
</div>
<img src="https://gis.ci.waco.tx.us/wacologoblacksmall.png" style="float:right">
And this is what you get...