how can we have a dial-in option in teams meeting invite created by shared mailbox

The easy answer is, it is not possible. But there are workarounds or proper way to go.
Proper way would be converting shared mailbox to full user account with assigned Teams license and Audio Conferencing license. It would cost you more for sure.
Easier way forward is to creating Teams, using channel meeting, it will have also calendar etc. and if you are using on your account Audio Conferencing license it will create meeting on behalf which means the meeting will be provisioned with dial-in information.
You might try also this way...but I do not see it as fully working workaround
https://techieberry.com/teams-meeting-from-shared-mailbox/