I know you can set a field value using a URL argument like this:
Is it possible to set the value of a null field? The entry point to the survey uses a null field value to set relevance for the next page in the survey. I need to set it to establish that via the URL.
Also, is it possible to set the page using a URL argument like:
Yes it is possible to pass a value via a URL into a field that has a bind::esri:fieldType of 'null'. This should work just like any other field, using the field:fieldname= syntax. While this field does not exist in the survey's feature layer, it does 'exist' for the purposes of the survey form.
Regarding pages, it is not possible to set a page number via a URL parameter. You can, however, hide pages using the hide= parameter.
Please find some great info on web app URL parameters in this blog post https://community.esri.com/groups/survey123/blog/2019/02/06/survey123-tricks-of-the-trade-web-form-u... and here Integrate with other apps—Survey123 for ArcGIS | Documentation.