how do i write a web service [WebMethod] to accept a string value from a textBox in an Infopath form.

you cannot access a textbox directly from a web service. The web service and the application invoking the web service are in no way related. What you can do is to pass the value of the textbox to the web method.

Regards
Ganesh

Create a method on your web service page marked with the WebMethod method attribute. Have this method accept a string as an argument and your web service will accept a string. Like Ganesh said, how you consume this web service is separate. You can't post a textbox to a web service.

-Peter