Howdy B4SsFI3ND,
Thanks for participating in the Apple Support Communities.
When you're setting up a reminder, have you tried saving it to see if you can then edit and add the reminder while messaging? In addition, are you aware if you have any restrictions or profiles installed on your iPhone that may be preventing this from occurring?
Check out the steps below to confirm this is the same process you're using to get a reminder while messaging someone. You can locate additional information here: Use Reminders on your iPhone, iPad, or iPod touch - Apple Support
Get a reminder while messaging someone
1. Tap a reminder, then tap the Edit Details button
.
2. Turn on When Messaging.
3. Tap Choose Person, then tap a name in your contacts.
The next time you chat with this person in Messages, a Reminders notification appears.
If you're unsure about any configuration profiles installed on your iPhone, you can go to Settings > General > Profiles & Device Management to check that. For more information on this, take a look here: Install or remove configuration profiles on iPhone - Apple Support
We hope that information is helpful. If not, you can Contact Apple Support directly to check into this with you in more detail.
Take care!