Use hashtags and then a space for bold.

One Hashtag followed by a space looks like this.

Two like this

Three like this

Four like this

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TO center, put a "< center >" before you text (But take the spaces out)

And put a "</ center>" after your text (But take the space out.)

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Then it will look like this

Or combine it with the BOLD HASHTAG Trick to make it look like this

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Give it a try in a reply here and see what you can do.