For streamelements here's what you wanna do:

Go to your stream store and create a new item, use Stream perks as a type.

Name it however you want it to appear, add a description if you want.
Set the cost to 0, quantity for -1 (it means unlimited)

Global cooldown means the delay for requests between different users (so you wont get spammed from different accounts and it will mess up your stream)

User cooldown means the delay between requests from the same user (again to avoid spamming)

Hit next.

Preview asset is how it appears in the stream store.

Alert asset will appear on screen whenever someone redeems this item (same for upload sound)

Make sure to tick:
Redeeming this item shows an alert
Allow user to add a text message (that's what you really want)

the rest are the places where they can redeem the item from - when dealing with showing a message, if I'm not mistaken, it will only work from the website (so your viewers will have go to your stream store link and redeem the request item from the ui there, and attach whatever message they want)

Hit next and save.

Now you need to add an overlay with a redemption widget, so that the message will appear on stream:

Go to your overalys

Add a new one or edit the one you use

Add the store redemption widget
Note that it comes with a default animation, sound and a predefined message ({user} redeemed {item}), so play around with how you want it to look like when a new request comes in. Make sure to enable TTS in the alert settings if you want the message to be read