Hi there, and welcome to community.rstudio.com! It's a little difficult to answer this question without seeing your code or understanding your use case, but it sounds to me like you may want to add either a numericInput() or textInput() to your ui logic, then reference the input's inputId in your model in the server logic. You may find the Shiny basics article helpful in understanding how to implement this.
If the above doesn't answer your question, can you please add a little more information as well as a reprex (reproducible example)? This will ensure we're all looking at the same data and code, and that we know what you're trying to accomplish. A guide for creating a reprex can be found here.