This is not possible; YouTube comments only support very basic formatting, as described e.g. here or in this video:
Add formatting to comments
Add style to your comment
You can use rich text to format your comment with common special tags, such as:
*bold text* → bold
_italicized text_ → italics
-strikethrough text- → strikethrough
In your spreadsheet the issue is that the hyperlink as well as your named range DATE_RATES are dates.
In this case you do not need the & but the DATEVALUE function.
Please use the following formula:
=INDEX(GBP_RATES, MATCH(DATEVALUE(A1), DATE_RATES, 0))