Welcome to Music: Practice and Theory Stack Exchange. Your question is one I get often from folks in the beginning stages of learning to play guitar.
You are absolutely correct in stating that knowing just the chords is not sufficient information to play the song in an authentic manner. The guitar (especially if played as a solo instrument) is a rhythm instrument and the strumming pattern is a big part of what establishes the rhythm for the song. So it is very important information.
There is actually no such thing as an "official strum pattern" (see this answer on Music Practice and Theory to learn more) Guitar Notation and Strumming Pattern Indicators.
And while you can often find chord charts and/or tab for some songs which do indicate a strumming pattern, or find a tutorial on You Tube that shows a suggested strumming pattern, these are still only someones interpretation and suggestion. Unfortunately, not all songs have a lesson or tab that includes a strumming pattern suggestion.
In the absence of any online lesson or YouTube video or Tab or chord sheets that indicate a suggested strumming pattern, you might need to develop your own strumming pattern!
There will be more than one variation of a strumming pattern for almost any song that will work for that song. The idea is to approximate the rhythm and feel for the song you want to learn, as best as possible. To do this, you should find a recording of the song that you like and that has as distinct rhythm that you can hear. Maybe in the bass line or the drum pattern or the guitar. Once you have selected a good recording to try to emulate, listen to it a few times, then try playing along with your guitar.
Experiment with different up down/down up/down down up/etc. patterns until you find something that you can play that keeps the same rhythm as the recording (or something that works). It's a trial and error process.
Personally, I just try to find my own strumming pattern based on what I hear in the song and what feels right. This ability to feel the rhythm and approximate it into a strumming pattern, will develop over time as you continue to learn more songs. If I try to follow someone else's strumming pattern, I end up either getting confused, or deciding that theirs is not as good as what I hear and feel.
One other thought. Even if there is not a YouTube tutorial that teaches the exact strumming pattern, if you can find a video of someone covering the song (the song in your question will have hundreds) where you can see the guitar player, just watch what he/she is doing on the strumming. You might have to sift through a few covers to find one that sounds good to you and that you can see what the guitarist strumming hand is doing.
Good luck with this and with your journey towards a lifetime of pleasure and fun playing guitar.