@Catherine Thorpe When you go to recordseek.com you will see the mint green rectangular button with 'RecordSeek' in it and you click and drag it to your toolbar/bookmark bar. Then when you are on a source you want to attach to FamilySearch.org click on RecordSeek and it will create the source and you add the PID of the person you want to attach the source to. It's a great tool! Let me know if you need more direction.
Not so long ago I added "RecordSeek" as "Extensions" to the 'Toolbar' of, BOTH, the "Browsers" of, 'Google' "Chrome"; and, 'Mozilla' "FireFox", where I am using "Windows 10".
There was no problem/issue doing so.
You "Add" that particular "Extension" through/via the "Browsers" themselves.
Are you having a problem/issue with adding "RecordSeek" as an "Extension" to the 'Toolbar' of your "Browser"?
IF, so; THEN, what, "Browser"; and, what "Operating Systems" are you using?
I hope this helps.
ps: A link to your other "Question' of the same.