Is there a way to verticaly occupy all the space of the flexbox container?

I want the search button to occupy all the space vertically I have tried setting the height to 100% of the parent container but not working, I have tried setting the flex-basis to 100%/0/auto (not working). Look at my Code below and see the picture of the btn beside the input.

I want that btn to gain the height equal to the height of the input

See the Problem here

My Html:

<div className="search_subContainer">
      <input
        className="searchCity"
        type="text"
        value={value}
        placeholder="Search city..."
        onFocus={() => {
          setFocus(!focus);
        }}
        onBlur={() => {
          setFocus(!focus);
        }}
        onChange={handleChange}
      />
      <button className="search_btn">
        <IoSearch />
      </button>
    </div>

My Css:

.header .search_subContainer {
  display: flex;
  align-items: center;
  align-content: stretch;
  height: 100%;
  border: 2px solid brown;
  border-radius: 5px;
}

.header .searchCity {
  width: 17em;
  height: 100%;
  padding: 0.3em 0.5em;
  font-size: 0.9rem;
  border: none;
  outline: none;
  background-color: rgba(255, 255, 255, 0.377);
  font-family: var(--secondary-font);
  border-top-left-radius: 5px;
  border-bottom-left-radius: 5px;
}

.header .searchCity:focus {
  background-color: rgba(255, 255, 255, 0.719);
}

.search_btn {
  width: 3em;
  flex: 1;
  height: 100%;
  background-color: brown;
  border: none;
  font-size: 0.9rem;
  border-top-right-radius: 1px;
  border-bottom-right-radius: 1px;
  cursor: pointer;
}

.search_btn:hover {
  background-color: cadetblue;
}