Javascript into html

Hello,
I've been trying to add this javascript of my Modern Events Calendar shortcode

It looks like this
<script type="text/javascript" src="https://dancevents.co.uk/mecExternalScript?id=148&t=calendar"></script> (doesn't work)

I have tried - <p><script type="text/javascript" src="https://dancevents.co.uk/mecExternalScript?id=148&t=calendar"></script></p> (doesn't work - and it disappeares after I click save)

Same code as above I've managed to embed on a test website of mine and it works

From a different app I have tried :
<p><iframe style="display: block; width: 280px; height: 491px; border: none; border-radius: 10px; background: transparent; margin: 0 auto;" src="https://snkt.io/62b58ba6ca4f1dc712712a1c"></iframe></p>
this one works like a charm

Would you be able to help?
Thank you
Cosmin