Hey @Muhammed_A,
welcome to the KNIME community! 
There is no direct option in the Parameter Optimization Loop to use String values, but you could encode your Strings as numerical values. Let’s say you have 10 String values; then you would add a new parameter to the parameter optimization with a range from 0 to 9 and step size 1. Afterward, you use the flow variable that has been created for the current iteration to filter rows in a given table, which maps string parameters to integers. Doing so, you can select the string value in the desired learner node.
I created a simple workflow to demonstrate this.
StringParameters.knwf (320.9 KB)
If you have more questions, feel free to ask!
Best,
Julian