EpiServer Forms: TextboxElementBlock how do I add attributes like maxlength?

I want to add maxlength to my input textbox (I use epiServer Forms 4.2.1), but I can't do this in epiServer CMS.

There is an option to make this work? There is a clue in the code that it is supported:

<%: html.raw(model.attributesstring) %>%:>