Send email to requester after new user request

Send email to requester after new user request

Hi,
As part of a new user request, want to initiate an email to the requester if they have chosen a field on the form (requesting a Mobile Phone). Would I do this under Field & Form Rules (on submit) or under Workflow (create a Task and action it). In either case, I cannot see how to send a preformed email with the WORKORDERID as part of the subject
Thanks
Chris White

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