Make a single line of text bold in Mail message using Automator?
The following example AppleScript code is one way of achieving your goal:
set clipContent to (the clipboard as text)

set lineOfBoldText to &quot;Line of text &lt;---- I want this line in **BOLD**&quot;

set myContent to linefeed &amp; &quot;Line of text&quot; &amp; linefeed &amp; linefeed &amp; ¬
    clipContent &amp; linefeed &amp; linefeed &amp; linefeed &amp; lineOfBoldText &amp; ¬
    linefeed &amp; linefeed &amp; linefeed &amp; &quot;Regards,&quot; &amp; linefeed &amp; &quot;Name&quot;

set x to offset of lineOfBoldText in myContent
set y to x + (length of lineOfBoldText)

tell application &quot;Mail&quot;
    set myMessage to make new outgoing message at ¬
        the end of outgoing messages with properties ¬
        {subject:&quot;subject&quot;, content:myContent}
    tell myMessage
        make new to recipient at ¬
            end of to recipients with properties ¬
            {address:&quot;email@email.com&quot;}
        set font of characters x thru y to &quot;Helvetica Bold&quot;
    end tell
end tell


Having copied to the clipboard &quot;In this message, I would like a single line in bold as seen here:&quot; from your OP and run the example AppleScript code, shown in my answer, in Script Editor, here is a screen shot of the results: