Please login your telnyx account, then click the "Real-Time Communications > Voice > SIP Trunking". Please select the "Credentials" type when you create SIP connection as the below image shows.

You can choose the default settings for other steps. It will auto create a username and password for you. You can also change it later.  Please copy these information to Ecsow dialer as the below image shows. Please notice the "Domain" field should be sip.telnyx.com .