[otrs] Custom field

We are working on linking an external CRM system to OTRS tickets. We were planning to use CUSTOMERID as the link field but this is automatically populated with the email address of the customer who submitted the ticket. Therefore we need to create a CUSTOM FIELD in OTRS that our customer service agents can populate with information (specifically the phone number of the customer) which we will then use to link to our CRM system. How, step by step, can I create a custom field in OTRS? Thanks in advance.