Hi @Anonymous ,
Do you want to launch your standalone canvas app via the "+New" button in your SP List?
If you want to launch your standalone canvas app via the "+New" button in your SP List, I afraid that there is no way to achieve your needs in PowerApps currently.
In addition, the created canvas app could not be resued for multiple SP Lists at same time.
If you would like this feature to be added in PowerApps, please consider submit an idea to PowerApps Ideas Forum:
https://powerusers.microsoft.com/t5/Power-Apps-Ideas/idb-p/PowerAppsIdeas
As an alternative solution, I think the "Customize Forms" option within SP List could achieve your needs. On your side, you could consider custom a form in your SP list using PowerApps via pressing "Customize Forms" option, then when you click "+New" button, it would launch your custom form to enter data.
Note: Currently, the "+New" button in SP List could only be integrated with Custom form functionality rather than standalone canvas app in PowerApps.
Please refer to the following article to custom a form in your SP List using PowerApps:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/customize-list-form
Best regards,