How can i use email channel to create new ticket and fill the mandatory ticket field by email?

Hi Fala,
 
It's possible to do this via Triggers, with some limitations:

• You can only set Checkbox, drop-down, and date custom fields via triggers, so you can't, for example use a trigger to set a text field.
• You'll need to create a trigger for each field value – so for example, if you have a dropdown field with three values (Value1, Value2, and Value3), you'd need to create a separate trigger for each of the three values.

 
If you're just getting started with building your own triggers, here are some useful resources: Trigger Resources

Hi Fala,
 
That's the Mail API – agents can use it to update certain ticket fields, but it's not available to end-users, and Text fields are not an option there either: Using the Mail API to update ticket properties from your inbox