You will need the Limited Write Forms permission to edit a form. If your account has more than one organization, the opt-in gives consent to texts from the organization that owns the form, or from your default organization when the form is not owned by exactly one active organization.
Any form that collects a phone number can ask for text messaging consent alongside it. This is the route most churches do not know they have, and it is the best one after your public opt-in page: it grows your reachable list from the connection cards, event registrations and interest forms you already run.
It is also stronger evidence than marking someone opted in yourself, because the person answers the question in their own words on your form.
Turn the question on
Open the form and go to its Fields.
Click on the Submitter Profile field or the person field.
Under Phone, turn on Show text message opt-in?.
Click Done, then save the form.
What the person filling in the form sees
Nothing, at first. The question stays hidden until they have entered a 10-digit number and set the phone type to Mobile. It then appears under the phone field, worded like this:
"Are you interested in opting in to receive text messages from [Church] at the number provided above? Reply STOP to [messaging number] to cancel. Message and data rates may apply."
It is answered Yes or No, not a tick box. Yes subscribes the number. No records an opt-out, which your church can reverse later. Leaving it unanswered does nothing either way.
Two things to know before you turn it on
It does not force profile creation. You can turn the question on and still leave Create new profile if submission cannot be matched to an existing person profile? off. This suits forms that collect someone else's number, such as volunteer references, where you do not want a new profile created for every submission.
It needs a mobile number to appear. If the question never shows, the submitter almost certainly left the phone type on something other than Mobile. The question is also hidden if your church has not enabled text messaging or does not yet have a messaging number.
How the consent is recorded
Consent granted this way appears on the Text Opt Ins screen with a Consent Source of Set On Profile, the same value a staff member marking someone opted in produces. The column cannot tell the two apart, so keep the form entry itself: that is your evidence if a carrier ever asks.
Tip: If someone insists they answered Yes on your form and their row still reads Texted STOP, the form entry saved but the consent was refused. Someone who stopped your messages themselves can only return by texting START or UNSTOP to that messaging number.