Serilog ExpressionTemplate rename log level and apply formatting

I'm trying to use serilog's ExpressionTemplate and write the log level in the following way:

{ "level": "INF", ... }

I know how to alias @l to level - level: @l and I know how to format the level to three upper-case letters - @l:u3 but I'm unable to combine the two

I have tried the following without success (fails when trying to format the template with an exception):

level: @l:u3

{'level': @l:u3}