Intent

You can use an intent to directly launch the player of mpv-android.

package: is.xyz.mpv
action: android.intent.action.VIEW

If you need to force an URL to be opened in mpv regardless of the file extension set the MIME type to video/any.
extras: (all optional)

Examples
Kotlin
val intent = Intent(Intent.ACTION_VIEW)
intent.setDataAndType(Uri.parse("https://example.org/media.png"), "video/any")
intent.setPackage("is.xyz.mpv")
val subtitle = Uri.parse("https://example.org/subtitle.srt")
intent.putExtra("subs", arrayOf<Uri>(subtitle))
startActivity(intent)
HTML (Chrome)
<a href="intent://example.org/media.png#Intent;type=video/any;package=is.xyz.mpv;scheme=https;end;">Click me</a>
Command line (e.g. adb or Termux)
am start -a android.intent.action.VIEW -t video/any -p is.xyz.mpv -d "https://example.org/media.png" 

Result Intent

Once the activity exits mpv-android will deliver a result intent back to the invoker.
action: is.xyz.mpv.MPVActivity.result
code:

data: the same URI mpv was launched with
extras:

Notes

This API was inspired by the counterpart in MXPlayer.
Note that only Java code is powerful enough to use the full features of this specification or receive result intents.