Hi @julienvdc
In case of instant cloud flow, you can configure the flow to run from a particular connection. It will applies to flows call from power app. If you are using automate flow then make a child flow which itself is an instant cloud flow and call it with the inputs to create an item in SharePoint site.
Please follow the below instructions:
1. In instant flow, go to the 'Run only users' setting as shown below:

2. Next, scroll down to connection used section at the bottom:

here am using Office 365 group connector in your case it might be SharePoint.
3. Next, choose use this connection option from the drop down:

4. Finally, click ok and save.
Please try using the abovesaid steps & let me know in case of any issue.
Please remember to give a 👍 and accept my solution as it will help others in the future.
Thanks