Log into your account on Sprint, and you should see a blue line offering to allow you to add a line for $0 per month.
Click through and choose the Samsung S20FE.
You will then be asked if you have a trade in. Select no, and you will see an offer to get the phone for $29.17/Mo, with a $29.17/Mo recurring credit.
Add the phone to your cart and you are all set.
For some reason, there was a glitch in the system when I did it, and I had to call phone sales to complete my order. I spent a lot of time with the rep, and ultimately her supervisor confirming that A) I would not pay the $20/Mo line fee, and that B) I would receive the $29.17/Mo credit