SendTmpToRenderTexture.cs

[SerializeField] TextMeshPro tmpStandard;
[SerializeField] TextMeshProUGUI tmpUI;

public void SendTmpToRenderTexture()
{
    Vector2 size = new Vector2(100f, 100f);
    RenderTexture tex = RenderTexture.GetTemporary(100f, Mathf.RoundToInt(size.y * 200f), 24, RenderTextureFormat.ARGB32);

    GL.PushMatrix(); // copy current camera matrix settings
    GL.LoadIdentity();

    Matrix4x4 proj = Matrix4x4.Ortho(-size.x / 2f, size.x / 2f, -size.x / 2f, size.y / 2f, -10f, 100f);
    GL.LoadProjectionMatrix(proj);

    RenderTexture currentActiveRT = RenderTexture.active;
    Graphics.SetRenderTarget(tex);

    GL.Clear(false, true, new Color(0f, 0f, 0f, 0f));

    if (tmpUI)
    {
        tmpUI.materialForRendering.SetPass(0);

        Graphics.DrawMeshNow(tmpUI.mesh, Matrix4x4.identity);
    }
    else
    {
        if (Application.isPlaying)
            tmpStandard.renderer.material.SetPass(0);
        else
            tmpStandard.renderer.sharedMaterial.SetPass(0);

        Graphics.DrawMeshNow(tmpStandard.mesh, Matrix4x4.identity);
    }

    //restore settings
    GL.PopMatrix();
    RenderTexture.active = currentActiveRT;

    //assign texture to image
    img.gameObject.SetActive(true);
    imgRt.sizeDelta = thisRt.sizeDelta;
    img.texture = tex;
}