TXT or text records are unformatted text usually with a host or other name. The most common uses for TXT records are Sender Policy Framework (SPF), DomainKeys (DK), and DomainKeys Identified E-mail (DKIM). TXT records historically have also been used to contain human-readable information about a server, network, data center, and other accounting information.
Please note if you have a TXT value that is longer than 255 characters, you will have to split the value into 2 parts using double quotations "". You will have to add both parts as your TXT record value.
For Example:
"String One""String Two""String Three"
1. Select your Domain
A. Select Managed DNS
B. Select Domains from the dropdown 
C. Select the domain you want to add a TXT record too.
2. Select TXT Record
Under the TXT records section, click the (+) icon to add a record.
3. Enter Record Values
A. Name: The hostname for the record.
B. TTL: Time to Live, measured in seconds, determines how long the record is cached in resolvers. For more information on best practices for TTLs, click here.
C. Value: The DKIM, SPF or other string for your domain/subdomain.
D. Notes: Add a helpful note with keywords so you can search for your records later.
E. Save: Save your record changes and don’t forget to commit your changes after you’re done making record changes for this domain!