For Infinity Studio (26.1) the configuration of the Application Signature will be a bit different than the provided instructions from Pega Marketplace ( Pega Application Signature | Pega ). For you that are asking, what is the Application Signature tool here is a great Expert Circles Link: Start your modernization journey with confidence with the Application Signature Tool today.
The 1st slight change in the installation instructions will be in the 2nd step.
To add the Application Signature ruleset to your application, select your Application Name at the top Right and then select View Application Rule.
The 2nd change is how to quickly find your access group to add the Pega Application Signature portal. For me the fastest way was to select Application > Records > Security > Access Group
The last difference is in how you will now access the portal. After you have selected the 
icon, which will now open a new preview window (great new addition to Infinity Studio), select the UI Designer Icon at the top of the screen 
. UI designer will allow you to select the Pega Application Signature portal that you added to your access group.
