Postpaid iPhone 12 - converting physical sim to eSim

I've been a postpaid customer for over 10 years. I'm currently using my iPhone 12 with a physical sim card. Whenever I travel internationally, I remove my t-mobile sim, and insert the local sim in the country I'm traveling for local use until I return back to US. I understand that iPhone 12 can support both physical and eSim. My question is: How do I move my current physical sim from T-Mobile to eSim so I can keep the sim slot free for using when I travel internationally? The menu 'Settings→ Cellular' doesn't show the transfer to eSim option. 

Gracias