To add multiple values together using the simple text method, only use the ? at the beginning and then separate each parameter value pair with an ampersand (&).
#wm-countup?speed=3000&start=50
<center>
<h2 style="margin:0"
data-wm-plugin="countup"
data-speed="3000"
data-start="50">100</h2>
</center>