you can add field tooltips to Acumatica mobile screens by customizing the ASPX page.

            <!-- Basic field example with tooltip -->

            <Control Type="FormViewField" DataField="Description" ControlID="edDescription"

                    Label="Description" Required="True"

                    ToolTip="Enter a brief description of the expense" />

To implement this:

1. Go to Customization Projects
2. Create or open your customization project
3. Add a Mobile Site Map
4. Find the expense receipt screen
5. Add the ToolTip attributes to the desired fields
6. Publish the customization

Try to modify like this

I think the ask here was to add the field called tip.  Not a tooltip.

This needs to be added to the customization for the mobile:

update screen EP301000 {

  update container "Details" {

    add field "TipAmount"

  }

}