This would likely be a Text field with a character limitation of 24 characters and a "letter" validation. Each field you add to a document inside an envelope has field properties that can be set for that field, here is a DocuSign article that provides information on the field properties and the two items I mentioned (character limit and validation):

https://support.docusign.com/en/guides/ndse-user-guide-field-properties

Hi,

Thanks for your answer. What I mean is that the field need to have exactly 24 characters (not less, not more). If I set the character limit to 24, then it will let me entering less then 24 characters and it's not ok.

Hope you can help me with a solution for this.

Have a nice day,

Clara

Ok, that would require a "Custom" Validation of a Regular Expression. A simple regular expression (as seen below) like this would indicate exactly 24 characters, alphanumeric, uppercase and lowercase allowed. Mine is just an example but I tested and it did work, Regular Expressions are very powerful and can be used in many situations.

^[a-zA-Z0-9]{24}$

So you add the Text field you go to the right side field properties, then Validation, select Custom then add the RegEx and the error when the criteria is not met.

LEN(Field__c) < 24

*To clarify, you create a validation rule using the above formula. The 'Field__c' should be substituted with the API of the field that you are referring to.