My question is if the word before the quotation marks should have a comma after it, and also, should the word in quotation marks be capitalized. The sentence looks like this:
Together, let's automatically add "Mmm" to end of this speech.
The rule that says to use a comma before a quote and caps within a quote such as
And then she said, “Huh.”
only applies to quotes speech, and not to all uses of a quotation mark. In this context I would use no comma and no capitalization. Similarly, if the quoted word were to end the sentience, the period would go outside, not inside, the quotation marks.
For a discussion see here, which uses the example:
The phrase "lovely, dark and deep" begins to suggest ominous overtones.