There is no official standard for the kind of text-message QR code this page makes. The generator writes SMSTO:, then the number, then a colon and your message, for example SMSTO:+61412345678:BOOK. ZXing, the open-source barcode library, lists SMSTO: as a form it has "observed in the wild" and says a scanner should open a new message ready for the user to compose and send.
Why not the official sms: link. RFC 5724 does define an sms: scheme, with the message in a body parameter, and says opening one should not send a message without the user confirming. Phone makers document less. Apple's URL scheme reference describes sms: with a phone number only and says the link must not include message text. Android lists sms: and smsto: as schemes its messaging apps handle, and passes the message separately. Neither describes a message inside a scanned code, which is why the message part is a scanner convention and why a test on both kinds of phone comes before any print run.
Write the number with its country code. The generator keeps a leading plus and strips every other character that is not a digit, so +61 412 345 678 becomes +61412345678. It keeps any zero you type, though, so a number written as +61 (0)412 345 678 comes out with a zero in the middle that no phone can dial. Type it the way it is dialled from another country.
Keep the message short. The message goes into the code as plain text, so its length sets the grid. SMSTO:+61412345678:BOOK is 23 bytes, which fits a 25 by 25 grid. An 85-character request for a quote takes it to 104 bytes and 41 by 41. At 20mm, the Business card size, that still prints at error correction M, but not once a logo raises it to Q. One word per placement also tells you where each text came from: POSTER on one code and CARD on another arrive as the first message in each conversation. People can edit the word before sending, so count it as a guide.
No UTM tags here. The scan ends in the messaging app, with no page load for UTM tags to ride on, so web analytics never sees it. If you want a code you can measure in analytics, point a Link code at a page and tag it with the UTM builder.
Printing it. Pick the placement in the sizing panel and it shows the grid, each square's size and a scan distance of roughly ten times the width. The print guide covers contrast and paper finish, and the QR code reader shows the exact text inside your downloaded PNG. For a tappable text link on a website or in an email, the click-to-chat link generator builds sms: links in both spellings.