Manually enter "Where From" text for screenshot Metadata?
Along with the information window, a dialog can be put up to get the whereFrom text, with a little AppleScriptObjC (since a shell script has been posted in another answer) to add it.  Extending the Automator folder action in my answer to your previous topic, the replacement Run AppleScript action would look something like:

use AppleScript version "2.4" -- Yosemite (10.10) or later
use framework "Foundation"
use scripting additions

property useBinary : missing value

on run {input, parameters}
    repeat with anItem in input
        tell application "Finder"
            activate
            open information window of anItem
        end tell

        set response to (display dialog "Enter text to add to whereFroms:" default answer "" buttons {"Enter", "Skip"} default button 2)
        if button returned of response is "Enter" then
            addWhereFrom(text returned of response, anItem)
        end if
    end repeat

    return input
end run

to addWhereFrom(newItem, filePath) -- add to existing whereFroms, trimming duplicates
    set whereFroms to readWhereFroms(filePath)
    set end of whereFroms to (newItem as text)
    set whereFroms to (current application's NSOrderedSet's orderedSetWithArray:whereFroms)'s allObjects()
    writeWhereFroms(whereFroms, filePath)
end addWhereFrom

to readWhereFroms(filePath) -- get a list of whereFroms from the extended attribute
    set filePath to quoted form of POSIX path of filePath
    set attribute to missing value
    set useBinary to missing value -- keep track of which it is
    try -- get existing attribute as property list
        set attribute to (do shell script "xattr -p com.apple.metadata:kMDItemWhereFroms " &amp; filePath &amp; "  | xxd -r -p | plutil -convert xml1 -o - -") -- convert from binary
        set useBinary to true
    on error -- oops, not a binary plist, so try XML
        try -- skip error if no attribute
            set attribute to (do shell script "xattr -p com.apple.metadata:kMDItemWhereFroms " &amp; filePath)
            set useBinary to false
        end try
    end try
    if attribute is in {missing value, ""} then return {}
    # deserialize the list from the property list string
    set theData to (current application's NSString's stringWithString:attribute)'s dataUsingEncoding:(current application's NSUTF8StringEncoding)
    return (current application's NSPropertyListSerialization's propertyListWithData:theData options:(current application's NSPropertyListMutableContainersAndLeaves) format:(missing value) |error|:(missing value)) as list
end readWhereFroms

to writeWhereFroms(theList, filePath) -- set the extended attribute to a list of whereFroms
    set filePath to quoted form of POSIX path of filePath
    # serialize the list into a property list string
    set theData to (current application's NSPropertyListSerialization's dataWithPropertyList:theList format:(current application's NSPropertyListXMLFormat_v1_0) options:0 |error|:(missing value))
    set plist to (current application's NSString's alloc's initWithData:theData encoding:(current application's NSUTF8StringEncoding)) as text
    if useBinary is false then -- set XML plist
        do shell script "xattr -w com.apple.metadata:kMDItemWhereFroms " &amp; quoted form of plist &amp; space &amp; filePath
    else -- convert and set binary plist
        set bplist to do shell script "echo " &amp; quoted form of plist &amp; " | plutil -convert binary1 -o - - | xxd -p"
        do shell script "xattr -w -x com.apple.metadata:kMDItemWhereFroms " &amp; bplist &amp; space &amp; filePath
    end if
end writeWhereFroms


To use the above script by itself, just remove {input, parameters} from the run handler declaration, and replace the input variable or set it to your own list of file items, for example from a choose file dialog.