The vast majority of conferences will announce the start of submission by sending out a Call for Papers to a list of authors/delegates. The process for sending out a call for papers using the Communication hub is described here; this article gives a brief overview of suitable content.
Conference Details
Firstly, the call for papers should immediately make it clear what the conference is and who is organizing it. The key details of the conference should be immediately obvious. The Communication hub will automatically populate the header of the message with the long name, the submission URL, and the start and end dates of the conference. You can also use some macros from the list here to populate the text of your email with information directly from the system.
How to submit
The critical thing that many of recipients will want to know is how to submit a paper or an abstract to your conference. You can use macros to enter the submission URL, or you can simply provide the conference URL itself (e.g., https://xyz2026.exordo.com/).
Other useful information to include for potential authors would be the dates on which authors can submit an abstract, the topics or themes that they can submit to, and the formats that they can choose to enter their submission. If you have any special formats like workshops or panels, it would be a good idea to provide as much information as possible on how to submit to these.
Anything else?
If you have a rough idea about your conference schedule, or if you have plans for invited speakers, it would be well worth mentioning these as any kinds of special events (even social events) or guest lectures could help attract potential authors and delegates.
If your conference is partnered with an academic journal, or sponsored by an affiliate, these are all things you may want to include in the call for papers. What you include in the call for papers is entirely up to you and what you believe may help your conference attract attendees. So long as you've made sure to provide the crucial information on how to submit a paper or an abstract, everything else is up for grabs and you should take the time to maximize the appeal of your conference!