Is it safe to add additional values to a TXT record in DNS

Solution 1:

Most of the semi-common things that store values in TXT records at the root are smart enough to have some kind of identifier in the record, or just accept all the TXT records and just look through them for the value they want.

Common TXT types that with some identifiers. SPF TXT record will start with v=spf1. DMARC with v=DMARC1; Google site verifications will start with google-site-verification=.

From your link

TXT value: d365mktkey=abc123abc123abc123abc123

The d365mktkey= part is used to define this record is for dynamics.

Anyway, nothing should break by adding another TXT record.