What I would do is make a node.js (javascript) app which uses express.js to set up an api endpoint a lib such as noblox.js for logging into roblox and changing someone’s rank in group, and then host it somewhere like glitch.com.
From that point you can use HttpService
to make a request to your api which will change the group rank for the target user.
An example with the noblox.js lib and express.js:
const express = require("express")
var app = express()
const rbx = require("noblox.js")
rbx.cookieLogin("your cookie here (best wa is to create a dummy account for this and rank it in group)")
app.get("/rankInGroup", function(req, res) {
var groupId = req.query.groupId;
var role = req.query.role;
var userId = req.query.userId;
rbx.setRank(groupId, userId, role).then(() => {
res.send("Success")
});
});
Then you would make a request to http://yoururl.glitch.me/rankInGroup?groupId=4702227&role=30&userId=444077685
(Forgive any typos, I typed all that on mobile)