GET THE SELECTED TEXT

Use a keyboard command via System Events to copy the selection to the clipboard

tell application "DEVONthink Pro"
	activate
	
	tell application "System Events"
		keystroke "c" using {command down}
		delay 0.5
	end tell
	
	set theResult to the clipboard
	display dialog theResult
end tell

This is a normal copy. Use "x" to cut the selection.