For adding custom stopwords, you can either add a find and replace step in the script part of your analysis; or write a custom vectoriser as an option in the models > feature handling screen. Let me know if you need any further guidance on these two options.
For the second option (custom vectorizer) you can activate it on the screen below:
The scikit-learn doc has very useful examples of such custom vectorizer. For instance, they provide a CountVectorizer class with a stopwords argument. See: https://scikit-learn.org/stable/modules/classes.html#module-sklearn.feature_extraction.text