People at our workplace sometimes prefer to be called their middle name. However, to find them in the company email directory, a person has to know their legal first name. What is the best way to indicate both names when writing a story about such a person?
A common convention for indicating a call-name is to put it in quotation marks (or sometimes in parentheses) immediately after the legal given (first, in English-naming traditions) name. This works for nicknames and completely un-related call-names as well as middle names. For example:
I've long considered the possibilities of the Dwayne "The Rock" Johnson Cinematic Universe.
—Matt Miller, "Dwayne 'The Rock' Johnson's Longtime Producer Confirms All His Movies Are Connected", Esquire, Jul 23, 2018)
Born in New York City on October 27, 1858, Theodore "Teddy" Roosevelt was governor of New York before becoming U.S. vice president. At age 42, Teddy Roosevelt became the youngest man to assume the U.S. presidency after President William McKinley was assassinated in 1901.
In the case of middle name-as-call name, it's common to put the middle name in quotes after both first and middle legal names:
William Bradley "Brad" Pitt (born December 18, 1963) is an American actor and film producer. He has received multiple awards and nominations including an Academy Award as producer under his own company Plan B Entertainment.
—"Brad Pitt", Wikipedia
Emma Grace "Grace" Smith was born . . .
—representative obituary/genealogy entry (I don't want to link for privacy reasons, but any search for "grace grace smith" as a phrase (or "marie marie johnson" or other common middle and last names) along with the word obituary will turn up multiple examples)
This form (Firstname Middlename "Callname" Lastname) also shows up for nicknames sometimes, especially where the full name is probably completely unfamiliar:
Lawrence Peter "Yogi" Berra
—Entry for State Historical Society of Missouri
On the other hand, the form Firstname "Callname" Middlename Lastname is sometimes used when the call name is a nickname that is clearly derived from the first name:
JAMES "JIMMY" EARL CARTER Jr. was born in a small farming town called Plains in Sumter County, Georgia, on October 1, 1924.
—National Governors' Association website entry
COMPANION (AC) IN THE GENERAL DIVISION OF THE ORDER OF AUSTRALIA: Ms Catherine (Cate) Elise Blanchett For eminent service to the performing arts as an international stage and screen actor, through seminal contributions as director of artistic organisations, as a role model for women and young performers, and as a supporter of humanitarian and environmental causes.
—"2017 Queen’s Birthday Honours Announced", Arts Review, June 12, 2017 (note that this organization prefers parentheses over quotation marks; this is a style choice)
Again, these examples can be found searching for likely examples, such as "katherine katie grace" or "margaret maggie jane", both of which will find numerous obituaries.
None of this is set in stone; whether you use single- or double-quotation marks or parentheses, and whether you put the call name between first and middle, between middle and last, or between first and last (omitting the middle) are all style choices. I would recommend repeating the middle name when it is the call name, though, for clarity's sake.