Just add the key pixels with pure magenta color (255, 0, 255) or {1,0,1}: https://love2d.org/forums/download/file.php?id=23260

In this example glyphs are:

local glyphs = " abcdefghijklmnopqrstuvwxyz" ..
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ0" ..
    "123456789.,!?-+/():;%&`'*#=[]\""

Just cut the line that you need to use.