highlighting a Word in the Textfield (Rich)

here is a situation , where the concatenated data becomes User editable
text field with bouded values from other source.
If the data is 'null' then the "____________" is replaced , it is an indication for the user that need to be filled. the whole content is a single textfield,
where only "_______________" need to be higlighted. how to achieve this.

Eg:
This is to babak bala bala "______________" kasdjkljsadlkjasldkjslj ljd a
sldj sldjsldj sldjsl djsldjasl dsd sadasdas dasd ad asd.

Can anyone please help me !!

Thanks is advance.
regards