This is kind of complicated to explain, so I shall try my best.
It’s easiest to do this on Firefox or Safari- for Safari, you’ll need to turn the developer’s option on (by going to preferences, then advanced options, then checking the box at the bottom). For Firefox, you need to download firebug.
Go on to ‘Edit my Profile’ and right-click on the gender field, and select ‘Inspect Element’.
A new window will open with a bunch of coding. Right-click on the line that is highlighted, and select ‘Edit as html’.
I had female selected on my facebook, so I then copied the ‘<option value=”2”>Male</option>’ section of the code, and pasted it straight after itself.
In this new pasted section, change the ‘2’ to a ‘0’, and change ‘Male’ to whatever you want (it doesn’t matter what you change it to because it will just come up as Select Gender’).
After this, exit the Inspect Element window, select this new gender field, and save the profile changes. It should come up with ‘Select gender’, and facebook will use they/their etc as pronouns.
Any problems, don’t hesitate to ask.