ArcGIS Pro 2.8 API Reference Guide
Tooltip Property (PlugIn)
Example 

ArcGIS.Desktop.Framework.Contracts Namespace > PlugIn Class : Tooltip Property
Gets or sets a brief description of the command that appears in a small pop-up window when a user pauses the mouse pointer over an element, such as over a Button.
Syntax
public string Tooltip {get; set;}
Public Property Tooltip As String
Remarks

The Tooltip will by default return the tooltip text entered in its DAML declaration. Use this property if you need to change the tooltip at run-time.

Example
<button id="acme_rename" caption="Rename" className="RenameProjectItem" smallImage="/Images/GenericPencil16.png">
  <tooltip heading="Rename">Rename the project item.
    <disabledText>No project loaded.</disabledText>
  </tooltip>
</button>
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

PlugIn Class
PlugIn Members