Hi @raghwanibhupesh,
I made a short video to show how you can access additional characters/symbols when entering text.
You will see that when I start editing a piece of Mtext, I select the Symbol drop down from the Insert on the Mtest editor ribbon. If I don't see the character I need, I can slecet other, pick a font I want to pull from and then find the symbol. Once I have it selected, I can copy and then paste into my editor.
Depedning on the font you select, finding the charactermight take some hunting. Theta in the Arial font for instance is U+03F4

Alpha (lower case) is U+03B1

Please select the Accept as Solution button if my post solves your issue or answers your question.
John Vellek
Join the Autodesk Customer Council - Interact with developers, provide feedback on current and future software releases, and beta test the latest software!
Autodesk Knowledge Network | Autodesk Account | Product Feedback