Given a .env file in the working directory with lines in the format ENV_NAME=ENV_VALUE, it will assign these environment variables

.env

MY_ENV=MY_VAL
MY_ENV_2=MY_VAL_2
MY_ENV_3=MY_VAL_3
MY_ENV_WITH_EQUAL=MY_VAL=BROKEN

set_env_file.bat

SETLOCAL ENABLEEXTENSIONS :: This actually prevents environment variables from being modified and you should remove this line if you mean to use it
SET me=%~n0
SET parent=%~dp0

@ECHO OFF

:: Currently has an issue where if the value has an = in it (such as our secret key!) then it only gets the value up to the first =

FOR /f "tokens=1,2 delims==" %%a IN (.env) DO (
    SET "%%a=%%b"
)