You can do this in a workflow script against the sys_broadcast_message table (I'm actually working on the very same thing myself right now and just created the same).
Steps to accomplish:
1. create a role and assign it to the group that needs to see the broadcast message so you don't end up spamming all users each time (I created one called "approval_message" for mine).
2. In workflow editor, create a new workflow and use the "Run Script" node. Name it as you like.
3. Add the below script but modify it to suit your own needs:
var gr = new GlideRecord('sys_broadcast_message');
var date = gs.nowDateTime();
var end = gs.minutesAgo(15);
var incident = current.incident.number;
var sysID = current.sys_id;
var url = 'https://onedev.service-now.com/u_operational_mechanical_incident.do?sys_id='+ sysID;
var urlString = '<a class="web" target="_blank" href = "'+ url + '">' +incident + '</a>';
gr.initialize();
gr.message = 'Incident ' + urlString + ' has been submitted for a communication approval. Please refer to the My Approvals link to review the details of this incident and either approve or reject the approval';
gr.user_filter = 'approval_message';
gr.at_login= 'false';
gr.logged_in = 'true';
gr.email = 'false';
gr.notify_users_after_date = date;
gr.notify_users_until_date = end;
gr.insert();
I hope this helps!!
Please mark my answer helpful or correct, if applicable.
