You can't arbitrarily nest JSP tags like that, it's not legal JSP (or XML).
You need to set the value separately.
In this case I barely see a reason to special-case it in the rendered output, instead pass two parameters to enableTextboxDirect
. I don't see a usecase for doing it any other way.
<html:select property="distributionCategoryCode"
onchange="enableTextboxDirect(${model.editT}, '${model.dtxtrmks}')>
Even if you had to write a one-liner JS wrapper in case you cannot modify enableTextboxDirect
it's still a better solution than bothering to change the entire attribute.