You should use it with PropType imported from vue like Object as PropType<FlashInterface>:

import FlashInterface from '@/interfaces/FlashInterface';
import { ref,PropType, defineComponent } from 'vue';
import { useStore } from 'vuex';

export default defineComponent({
    props: {
        message: {
            type: Object as PropType<FlashInterface>,
            required: true
        }
    },
    setup(props) {
            // Stuff
        }
});

Note : you should create your component using defineComponent in order to get the types inference.

script setup

<Vue3.3 :

You could define the props using defineProps function in two ways, but you should declare the types/interfaces inside the script setup as mentioned here :

<script setup lang="ts">

import { ref,PropType, defineComponent , defineProps} from 'vue';
interface FlashInterface {
    level: string,
    message: string,
    id?: string
}
interface IProps{
   message : FlashInterface
}
const props = defineProps<IProps>()

or

<script setup lang="ts">
...
interface FlashInterface {
    level: string,
    message: string,
    id?: string
}
const props = defineProps({
        message: {
            type: Object as PropType<FlashInterface>,
            required: true
        }
    })

Update Vue 3.3+

Since Vue 3.3 (as april 2023 is in beta), the PR 8083

Set in your package.json under dependencies

"vue": "^3.3.0-beta"

Now the defineProps<T>() macro accept an imported interface and you can do this:

import FlashInterface from '@/interfaces/FlashInterface';
import { defineProps } from 'vue';

export default defineProps<FlashInterface>();