How can I solve a 403 error during app installation automation testing?

I need to write automation tests for our App installation , but first i had to be authorised as a user in Shopify, so when i try to get authorised through my tests script - I cannot do it because I have got 403 error all the time, so even the Login form is not opened, just error is displayed. So what can be the problem and how can handle this? Help me please with this, because it is a blocker for me.