When you go on to freebmd -
https://www.freebmd.org.uk/cgi/search.pl - you enter the name of the person you have, click on Find and you get taken to the details, i.e. Birth, quarter, year, the name and then the GRO reference details.
Click on the red info box. You get taken to another page and you will see the you may add a postem. Note that you only have 250 characters to get all the information in.
Explanation for Postem
A Postem is a note left by someone about this record - the content of the postem is determined by the person who left it. FreeBMD can offer no assurance about the postem and will not enter into correspondence about it. You can leave a Postem by clicking on Add a postem. Further information about Postems is available here.
Hopefully you can follow what I have said!