All you need is the correct html markup, namely a paragraph p
inside a container div
with a certain class. Usable classes are updated
green, error
& notice-error
red, notice-warning
orange, notice-info
blue. Exemplary usage:
add_action( 'admin_notices', 'wpse330943_admin_notices' );
function wpse330943_admin_notices() {
<div class="notice notice-error">
<p>
//some text
</p>
</div>
}