As the login fields are hard coded in myaccount/form-login.php template file, the only way to add placeholder(s) to those fields, requires overriding the related template via the active child theme (or active theme)
Once you have copied the myaccount/form-login.php template file located in WooCommerce plugin under the "templates" folder to (as explained on the template):
<?php
/**
* Login Form
*
* This template can be overridden by copying it to yourtheme/woocommerce/myaccount/form-login.php.
Open/Edit form-login.php copied file, and add to all desired <input> html tags:
placeholder="placeholder text"
Like for example replacing from line 38 to 45 with the following:
<p class="woocommerce-form-row woocommerce-form-row--wide form-row form-row-wide">
<label for="username"><?php esc_html_e( 'Username or email address', 'woocommerce' ); ?> <span class="required">*</span></label>
<input type="text" class="woocommerce-Input woocommerce-Input--text input-text" name="username" id="username" placeholder="<?ph esc_html_e("Here type your Username (or email address)"); ?>" autocomplete="username" value="<?php echo ( ! empty( $_POST['username'] ) ) ? esc_attr( wp_unslash( $_POST['username'] ) ) : ''; ?>" /><?php // @codingStandardsIgnoreLine ?>
</p>
<p class="woocommerce-form-row woocommerce-form-row--wide form-row form-row-wide">
<label for="password"><?php esc_html_e( 'Password', 'woocommerce' ); ?> <span class="required">*</span></label>
<input class="woocommerce-Input woocommerce-Input--text input-text" type="password" name="password" id="password" placeholder="<?ph esc_html_e(""Here type your password"); ?>" autocomplete="current-password" />
</p>
Then save… You will get something like:

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