The most straightforward way I found is by converting the string to a byte[] and returning that as ReadOnlyMemory, like so:

var memory = new ReadOnlyMemory<byte>(Encoding.UTF8.GetBytes(str));

Thee is a way. The EncodingExtensions class contains GetBytes extension methods that can write to an IBuffeWriter< T>. Two built-in classes implement this interface, ArrayBufferWriter<> and PipeWriter.

In APIs that use System.IO.Pipelines it's better to write to a PipeWriter directly rather than create an intermediate object. I'll use ArrayBufferWriter instead, because ... it's easier. It still allows memory to be reused instead of allocating new buffers:

var text=".....";
var writer=new ArrayBufferWrite(8192);

Encoding.UTF8.GetBytes(text,writer);

var memory=writer.WrittenMemory;

WrittenMemoy returns a ReadOnlyMemory<T> object with the written data.

The buffer can be cleared with Reset() and reused :

var writer=new ArrayBufferWrite(8192);
while(true)
{
    writer.Reset();
    var text=SomehowGetString();
    
    Encoding.UTF8.GetBytes(text,writer);

    var memory=writer.WrittenMemory;
    ...
}