example tiled scripts related to the addition of custom properties to Tiled Projects https://github.com/mapeditor/tiled/pull/3667

AddProjectProperties.js

var AddProjectProperties = {};

AddProjectProperties.action = tiled.registerAction("ProjectCustomProperties", function (action) {
    tiled.project.setProperty('newString', 'hello world');
    tiled.project.setFloatProperty('newRatio', 0.7);
    tiled.project.setColorProperty('newColor', "#aabbaa");
});
AddProjectProperties.action.text = "Add Project Properties";

tiled.extendMenu("Edit", [
    { action: "ProjectCustomProperties", before: "SelectAll" },
    { separator: true }
]);

ProjectReadProperties.js

var ReadProperties = {};

ReadProperties.action= tiled.registerAction("ProjectReadProperties", function (action) {
    tiled.project.property('myRatio')
    for (const propName in tiled.project.properties()){
        tiled.log(`Property ${propName} has value ${tiled.project.property(propName)}`)
    }
    tiled.log("Read properties")
});
ReadProperties.action.text = "Log all Project Properties";

tiled.extendMenu("View", [
    { action: "ProjectReadProperties" },
    { separator: true }
]);