Most of us are slowing migrating to power Automate flow from Workflows, Specially the BackGround Workflows because we know flow don't run synchronously,
But I was not even aware of on-demand Flow are available although we need default environment access to create this Power automate so it gets a little hard to ask connection permission from the client on the Default environment.
Another Great thing is Dialogs can be replaced with this Microsoft Flow Add User Input

Going to the flow.microsoft.com and no need to worried about Selecting the environment as mentioned above this only works for the Default environment.

• Also, don’t create this flow from solution as I also thought if I create it from our Dev CDS Solution then we can deploy it in any of the organizations but by doing this you wouldn’t be able to see the Flow from the entity

• Click on New Flow -> Instant-Blank flow -> Give it a Name and Select “When a Record is Selected”

• Only the Default Environment option will be an available and Run option available for all Environment available.
• We have a very interesting thing here that is taking input from the user example number email etc which can be used further in logic
• add actions steps as logic and save the flow

• Now Selecting any one or multiple Records and click on Flow, We will see our Flow created above
• if the flow is not visible there can be multiple reasons, please follow Check 3

• As we setup User input in our flow above, asking Number

• most common is we might have created Flow from CDS Solution
  Check out this blog from Jerry: where he showed us how  Solution and When a record is selected action Don’t get along
• Second is your flow might be Off, It happened to me
• Try clearing Browser Cache.

Hoping  this post was Helpful. thanks