i can only see options for notes from other maps but i want a custom written one, how would i do it?

You would need to make your own mod with mod override to have your own custom note content. The note can be placed by anyone making their own map into their map though...

https://docs.smartlydressedgames.com/en/stable/assets/asset-bundles.html#master-bundles
Would be a good place to start.
e.g. (For a normal note looking note with your custom written stuff)
For the object note's .dat file, include the following lines:
Master_Bundle_Override core.masterbundle
Bundle_Override_Path /Objects/Medium/Furniture/Note

That would make your custom note outwardly looking like a note.
Taking example from Note_0.dat, include the following as well:
Interactability Note
Interactability_Hint Use
Interactability_Text_Lines X (X being how many lines your custom text in English.dat have)

Taking example from Note_0's English.dat:
Name Whatever name you wanna give this so you can find easier in Map editor
Interactability_Text_Line_0 blah blah
Interactability_Text_Line_X-1 blah blah

There you go :)