Hello @Wiznamo,
I have also tried to save data directly to this table but never successfull.
When I had web application and was not using page activity (the one where you not insert HTML source). But this activity is needed to define variables that are saved in your recipient response form. I used to create test condition where 1=0 and true branch went to the page activity were the variables you want to save are defined. Then take a look at the context from web application and you will see these variables there, that you have created in the old page activity.
Try to simply populate them by
ctx.vars.xxx = 'your value'
and then at the end of the web application use the out of the box save activity which will save your custom input values.
Marcel