Add text to lock screen 7.1

You'd need premium to do this I think as you have to inject code into the lock screen. the standard custom css doesn't get inserted to the lock screen.

If you've got premium, go to Settings > Advanced > Code Injection

Then you can add something like this into the Lock Screen box (the bottom one)

<style>
.lock-screen::after {
    display:block;
    background-color:red;
    color:white;
    font-weight:bold;
    font-size:1rem;
    padding:1rem;
    margin:1rem;
    position:absolute;
    right:0;
    bottom:0;
    content:'Here is my red text'
}
</style>

this will add the block you want

EDIT: Assumed you've already tried this, but should point out that there are multiple editing and layout options for the lock screen in Design > Lock Screen and if you can use that to get your desired message across (albeit not in red and in exactly that position) you might find that easier.