How do I say this in my Gridview filter:

Do not show me {Cust #} that start with "TD".

Mid(Trim({Cust #}), 1, 2) <> "TD"

Trim() to remove leading blanks.
Mid() to return part of the text. The latter two parameters are position (starting at 1) and length.

Perfect. Thanks!

Hi Bob

Why use Mid rather than Left i.e. TRIM(LEFT({Cust #} ,2 )) <> "TD" ?

Just curious.

Either works, MID or TRIM. Just a preference.
Though you have to apply the trim first and then take the leftmost characters, otherwise the results might just be blank:
LEFT( TRIM({Cust #}) ,2) <> "TD"

MID is more universal...anywhere within the text. It's something I remember from the old BASIC programming language. Back when Bill Gates (& myself) was youthful.