Login to your 123Reg Control Panel. (https://www.123-reg.co.uk)
Search your Domain Name next to DOMAINS and Click on MANAGE.
Scroll down to Advanced Domain Settings and click on Manage DNS (A,MX,CNAME,TXT, SRV).
Click on the Advanced DNS tab.
Select TXT or CNAME as the Type and fill out the fields with the following info:
a) Hostname: Enter the host name for the TXT/CNAME record. For example, enter @ for an SPF record, or enter the value _dmarc for a DMARC record.
b) Destination TXT Value: The value you are setting as the destination of the Host.
When the information are entered, select Add.
Create a free OnDMARC account