Navigate to Mail flow in the Exchange Admin Center, > Connectors. The Connectors screen appears.
Click +Add a connector. The New connector screen appears.

Under Connection from, choose Partner organization.

Click Next. The Connector name screen appears.
Provide a name for the connector and click Next. The Authenticating sent email screen appears.
Choose one of the two options between By verifying that the sender domain matches one of the following domains and By verifying that the IP address of the sending server matches one of the following IP addresses, which belong to your partner organization.
Click Next. The Security restrictions screen appears.
Check the check box for Reject email messages if they aren't sent over TLS.
Check the check box for Reject email messages if they aren't sent from within this IP address range, and provide the IP address range.
Click Next. The Review connector screen appears.
Review the settings you have configured, and click Create connector.
Reference:
https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/set-up-connectors-for-secure-mail-flow-with-a-partner