processed.html?noInitialModGui=true
or inside eaglercraftXOptsdisables the mod gui display on startup
Set eaglercraftXOpts.Mods to an array of mod urls.
You can also use search parameters, eg:
processed.html?mod=https://example.com/mod.js
processed.html?mod=https://example.com/mod.js&mod=https://example.com/mod2.js