Adobe Sign Text Tag - Formatting numbers

I'm afraid you need to make use of regular expressions to get this validation to work. Also note it's a validation which is different than formatting the value of a field.

With validation the form filler is forced to enter the value in the right format

try this as a tag

{{#fein=*fein_es_:signer1:custom(regexp="^[1-9]\\d?-\\d{7}$",msg="Format: xx-xxxxxxxxx")}}

I got the regex from here https://regexlib.com/REDetails.aspx?regexp_id=1990&AspxAutoDetectCookieSupport=1

In the text tag an additional backslash has been added as per text tag reference guide;

In this syntax, the backslash char that is part of regexp constructs such as "\w", must always be escaped with a backslash., i.e., "\\w". And in order to search for a backslash, one must use \\\\. The string begin and end character double-quotes ("). If a quote is expected to occur within the data that the recipient enters, to provide a regexp validation string to match the quote they must be escaped as \" in the expression. A good reference for the syntax of regular expressions used by Adobe Sign can be found here.

For other users reading along,  the above tag is a reference tag used in text tag shortning. it requires another target tag in the main document along these line {{$fein   }}.

if not using text tag shortning the regula text tag would be {{*fein_es_:signer1:custom(regexp="^[1-9]\\d?-\\d{7}$",msg="Format: xx-xxxxxxxxx")}}