必須 and 任意: required fields and optional fields
Read required and optional form labels, distinguishing optional completion from 任意 used for an arbitrary choice of number.
A requirement that cannot be omitted
必須, read ひっす, means required or essential. 氏名の入力は必須です。 says that entering a full name is required.
必須項目をすべて記入してください。 asks for all required fields to be completed. Read すべて without reducing the requirement to just one field.
A field whose completion is optional
任意, read にんい, means optional in a form context. 会社名の記入は任意です。 says that entering a company name is optional.
電話番号は必須、会社名は任意です。 gives different requirements for the phone number and company name. Do not extend 任意 to permission to leave every field blank.
Choosing an arbitrary object
任意の数字を一つ選んでください。 asks someone to choose any one number. Here the number can be freely chosen; the request itself is not described as optional.
Separate an optional form field from an arbitrary choice of object. 記入は任意です and 任意の数字 use the same word with different targets.
May the company-name field be left blank under 電話番号は必須、会社名は任意です。?
Show the answer and why
Yes, under these instructions, because the company name is optional. The separate requirement to enter a phone number must still be followed.