install using a userscript manager like tampermonkey (may take a few minutes to get set up). can easily modify code to change the replacement character

pronto.user.js

// ==UserScript==
// @name         Pronto Underscore Messages
// @namespace    http://tampermonkey.net/
// @version      1.1
// @description  Replaces spaces with underscores in outgoing messages on Pronto
// @match        https://stanfordohs.pronto.io/*
// @grant        none
// @run-at       document-start
// ==/UserScript==

(function () {
  const originalOpen = XMLHttpRequest.prototype.open;
  const originalSend = XMLHttpRequest.prototype.send;

  XMLHttpRequest.prototype.open = function (method, url, ...rest) {
    this._url = url;
    return originalOpen.call(this, method, url, ...rest);
  };

  XMLHttpRequest.prototype.send = function (body) {
    if (this._url?.includes("/api/v1/message.create") && body) {
      try {
        const parsed = JSON.parse(body);
        if (typeof parsed.message === "string") {
          parsed.message = parsed.message.replace(/ /g, "_");
          body = JSON.stringify(parsed);
        }
      } catch (e) {
        console.warn("[Pronto Underscores] Failed to parse body:", e);
      }
    }
    return originalSend.call(this, body);
  };
})();