Modify

Keyboard Maestro supports use of JavaScript in two basic ways:
• Execute JavaScript in a Browser
• Execute JavaScript for Automation (JXA)

JavaScript, once a language which ran only in web browsers,
can now be used in a variety of environments.

From Keyboard Maestro you can:
1. Run it in Chrome or Safari web pages ('browser JavaScript'), or
2. Run it in Apple's osascript environment, for automating applications. ('JavaScript for Automation', or JXA, sometimes JSA in Apple's documentation),
3. Run it in Custom HTML Prompt windows.

https://wiki.keyboardmaestro.com/action/Custom_HTML_Prompt
Using JavaScript
You may use any JavaScript that you normally could in an HTML page.
Keyboard Maestro provides the following built-in JavaScript functions:
Function  Called 
KMInit()  to initialize fields or other features
KMWindow()  to return the desired size or frame of the window as a string
KMWillShowWindow()  when the window will be shown
KMDidShowWindow()  when the window was just shown (7.1+) 
From within the window, you can call the following functions:
Function  Purpose 
window.KeyboardMaestro.Submit( buttonName )  submit the form and write the Keyboard Maestro Variables
window.KeyboardMaestro.Cancel( buttonName )  close the window, do not write the Keyboard Maestro Variables
window.KeyboardMaestro.ResizeWindow( “1,2,3,4” )  resize the window to the specified size or frame as a string with commas (“width,height” or “left,top,width,height”) 
window.KeyboardMaestro.GetVariable( 'KMVariableName' )  returns the value of a variable 
window.KeyboardMaestro.SetVariable( 'KMVariableName', 'value' )  sets the value of a variable 
window.KeyboardMaestro.ProcessTokens( '%ShortDate%' )  returns the text token expansion 
window.KeyboardMaestro.Calculate( '10 + 32' )  returns the result of the calculation 
window.KeyboardMaestro.Trigger( macro, value )  triggers the specified macro (7.1+) 
Prior to version 7.1, using SetVariable to set a value to something other than a string can corrupt the variable.