Auto assign Org form contacts when created

What's the best way to auto assign the Org to a client when they email for the first time? We want to use their email domains to match to their email address, which is housed in a list on Site A.
We would also want our second help desk to do the same thing but to refer again back to to domain list on Site A.
How are Contacts created when a person registers/sends email? Is that process editable?

Hello Tricia!
The process of the contact creation can't be customised, but you can edit the newly created item right after that. For the automation, please use a Power Automate flow.

The "Organization" field in the "Contacts" list is a lookup column that refers to the "Organizations" list on the same HelpDesk site. Thus, you need to synchronize the latter list on each HelpDesk site with the common one on the "Site A" or just keep their data up to date. For synchronization, you can use another Power Automate flow.

So, to set the organization automatically for a newly created contact, create a Power Automate flow and run it when a new item is created in the "Contacts" list. Then, process its e-mail string to get only the domain component. Using it, find the accordant item in the "Organizations" list and assign it to the newly created contact.

Please let me know whether you need a detailed instruction on creating such a flow.