Adding new ligatures to an overleaf-document

I’m currently transcribing some old books using overleaf as my compiler of choice. I wanted to copy some old ligatures (æ, œ, ct, …).

I have already read some questions, (e.g. How add my own ligature) where it’s been said to use the fontspec-package (and XeTeX, of course) and include another mapping. Unlucky, this doesn’t work out on overleaf (or I failed myself) and now I’m wondering, whether there is another way to do so.

documentclass[11pt, doublespacing]{book}
usepackage{fontenc}
usepackage[greek, latin]{babel}
usepackage{alphabeta}
input{preamble} %(nothing too important)

usepackage{fontspec}[Mapping=ligaturae.map]

begin{document}

...

end{document}

I actually copied the map-file from the guide above (How add my own ligature) and added changed two lines to the custom ligatures I wanted

; additions by me
U+0061 U+0065   <>  U+00E6  ; ae -> æ
U+006F U+0065   <>  U+0153  ; oe -> œ