How do I force a text message to go over wifi when I have poor or no cell service but good wifi?

If your carrier supports some form of wifi-assist or texting via wifi calling, you should be able to send an SMS over wifi. However, if your account with your carrier does not support such a feature, then you cannot do so as natively, an SMS text requires a cellular voice channel for transmission. SMS is not natively an internet messaging service - it is a cellular voice messaging service.

You can also enable Wi-Fi Calling (and texting) in Settings -> Phone.

There is an option called wi-fi assist. You can check to see if that is enabled by going to Settings --> Cellular and scroll all the way to the bottom to see if that feature is enabled.