How to bold the text or change Color in mailto in Expression rule

Hi all,

I want to bold the text or change the color to red while replying from an email..

below anchor tag from the expression rule-

<a href='mailto:",

sample@mail.com,
"?subject=[",
ri!sample,
"]Sample Subject
[DestinationPMUUID=0000000-0000-0000-0000-00000000000]&body=
Sample Text %0d%0a %0d%0a %0d%0a

****I want this text in Bold'
target=_top>click here</a>

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