If the user can't interact, you have 2 ways:
use regular text
use a pre-filled, disabled field
In the first case, its main advantage is that the user can notice it's just a text and there's no affordance that suggest she can interact with that. However, if it's important, it might be overseen by the user
In the second case, it will be clear that this is ...
I would start with a 2-step version and then a/b test different registration flows in the live product.
Hypothesis to start with:
By splitting the flow into two steps, with each step having a more specific purpose, the completion rate will be higher.
Step 1: Essentials (email + password)
Step 2: Welcome (enter name + pick username)
Rationale for starting ...