Use a fixed-width font in iMessage extension MSMessage text

I'm developing an iMessage extension that makes some ASCII art out of text entered by the user. It needs to set the message text to a fixed-width font to look correctly. Does anyone know how to get MSConversation (insertText) or MSMessage to utilize a fixed-width font with a string?

Thanks.

Use a fixed-width font in iMessage extension MSMessage text
 
 
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