Hello @Sureka Harshani - Once you created online meeting using below API, you will get joinWebUrl value, which is the actual link to join a particular meeting.
https://learn.microsoft.com/en-us/graph/api/application-post-onlinemeetings?view=graph-rest-1.0&tabs=http#request
So, you can create your join URL based on joinWebUrl value via adding any HTML or other button with hyperlink or Action.OpenURL on card button..
Currently we don't have any such Graph API to directly join the online meeting.
You can raise your feature request here: Microsoft Teams Community
Could you please share your valuable feedback via Microsoft Teams Developer Community Response Feedback link?
Thanks,
Chetan Sharma
Dev Community Response Team