this is your KM macro as AppleScript (improve it if necessary):
-- LaunchBar Action Script
tell application "Google Chrome"
if (count of windows) < 1 then
make new window
else
tell window 1
make new tab
end tell
end if
activate
end tell
tell application "System Events"
keystroke "l" using command down
keystroke "bm\t"
end tell
Open the Action Editor
Create a new action, e.g. with the name “Chrome Bookmark Search”
Choose AppleScript as Default Script
Click Edit button
default.scpt will open in Script Debugger (or Script Editor)
Delete the content and paste the script from above
Save and close the script file.
Bring up LaunchBar, type cbs (or whatever corresponds to the name of your action), focus the action, press Return.
Repeat this once or twice to make LB learn your preferred abbreviation, or – if you’re really inpatient – force-assign an abbreviation with ⌥⌘A.
###PS:
If you prefer to type the search string in LaunchBar (instead of Chrome’s address field), then get the input string from the handler:
on handle_string(_string)
tell application "Google Chrome"
if (count of windows) < 1 then
make new window
else
tell window 1
make new tab
end tell
end if
activate
end tell
tell application "System Events"
keystroke "l" using command down
keystroke "bm\t" & _string
end tell
end handle_string
(I would probably prefer this, since it also allows you to send a text selection to the action (Instant Send). You can still modify your search in Chrome afterwards.)