Simple link from Dolphin file manager to file/folder on Dropbox.com using Maestral GUI under KDE. The .desktop file goes under `~/.local/share/kio/servicemenus/` and the script should be anywhere in your PATH, I used `~/.local/bin/`. Script depends on realpath, xdg-open, notify-send, and of course maestral.

maestral-open-dropbox.sh

#!/bin/bash
if maestral status | grep -qFi 'not running'; then
	err='Maestral not running'
elif ! droot="`maestral config get path`"; then
	err='Maestral not found'
elif ! real="`realpath -eLPq "$1"`"; then
	err='Path does not exist'
elif [[ ${#real} -le ${#droot} || "${real:0:${#droot}}" != "$droot" ]]; then
	err="The path $real is not below the Dropbox root directory $droot"
else
	if [[ -d "$real" ]]; then
		pfx=home
	else
		pfx=preview
	fi
	xdg-open "https://www.dropbox.com/$pfx${real:${#droot}}"
	exit
fi
notify-send Error "$err"

maestral-open.desktop

[Desktop Entry]
Type=Service
X-KDE-ServiceTypes=KonqPopupMenu/Plugin
MimeType=inode/directory;application/octet-stream;
Actions=maestralOpenOnDropboxSite

[Desktop Action maestralOpenOnDropboxSite]
Name=Open on Dropbox website (via Maestral)
Icon=link
Exec=maestral-open-dropbox.sh %f;