Although the context may be enough to figure out, the mention inspector is providing an input field without an explicit indication on what to input there:
As illustrated in T92588#1115945, having a placeholder in the input field can add some clarity. The above example with a placeholder:
Change 209623 had a related patch set uploaded (by Catrope):
Add placeholder for input field in mention inspector
Change 209623 merged by jenkins-bot:
Add placeholder for input field in mention inspector
Checked in Chrome, FF, and Safari. There are couple of issues unrelated to this one - but it seems that they have been addressed in T93421(still in Code Review)