Access team meeting chat before the meeting starts when created using API

Shubham5 - Thanks for reporting your issue. Could you please let us know which API are you using? Is it event API or online meeting API?
In order to create meeting in calendar you have to create event as an online meeting using below API

example:

POST https://graph.microsoft.com/v1.0/me/events
Prefer: outlook.timezone="Pacific Standard Time"
Content-type: application/json

{
"subject": "Let's go for lunch",
"body": {
"contentType": "HTML",
"content": "Does noon work for you?"
},
"start": {
"dateTime": "2017-04-15T12:00:00",
"timeZone": "Pacific Standard Time"
},
"end": {
"dateTime": "2017-04-15T14:00:00",
"timeZone": "Pacific Standard Time"
},
"location":{
"displayName":"Harry's Bar"
},
"attendees": [
{
"emailAddress": {
"address":"samanthab@contoso.com",
"name": "Samantha Booth"
},
"type": "required"
}
],
"allowNewTimeProposals": true,
"isOnlineMeeting": true,
"onlineMeetingProvider": "teamsForBusiness"
}

Ref Doc: https://learn.microsoft.com/en-us/graph/api/user-post-events?view=graph-rest-1.0&tabs=http#example-4-create-and-enable-an-event-as-an-online-meeting