How to write a service to get some string? I want to write a service on macOS, and it only return a string, like my email address. So in any application, if I need input email address, I just hit the same shortcut. 

I write a service in automator, and run shell script:

#!/usr/bin
echo "my_name@domain.com"

but it only output the text to stdout instead of application input.
You can add a "run applescript" command to your automator workflow

set the clipboard to "my_name@domain.com"
tell application "System Events"
    keystroke (the clipboard)
end tell


OR

tell application "System Events"
    keystroke "my_name@domain.com"
end tell




You can add this AppleScript to your Automator workflow to be able to insert the time and date from your clipboard

set AppleScript's text item delimiters to ","
set theLongDate to (current date)
set theLongDate to (date string of theLongDate)
set currentMonth to (word 1 of text item 2 of theLongDate)
set currentDay to (word 2 of text item 2 of theLongDate)
set currentYear to (word 1 of text item 3 of theLongDate)
set monthList to {January, February, March, April, May, June, July, August, September, October, November, December}
repeat with x from 1 to 12
    if currentMonth = ((item x of monthList) as string) then
        set theRequestNumber to (text -2 thru -1 of ("0" &amp; x))
        exit repeat
    end if
end repeat
set currentMonth to theRequestNumber
set currentDay to (text -2 thru -1 of ("0" &amp; currentDay))
set theShortDate to (currentMonth &amp; "/" &amp; currentDay &amp; "/" &amp; currentYear) as string
set CurrentTime to (time string of (current date))
set CurrentTimeandShortDate to (theShortDate &amp; " @ " &amp; CurrentTime)

set the clipboard to the result -- the result formatted like this 04/16/2017 @ 12:27:00 AM

-- If you only want to copy the time and date to your clip board without sending the keystrokes, Then comment out the next three lines

tell application "System Events"
    keystroke (the clipboard)  
end tell