How to Enable Sign in with Shop in Shopify Force Theme

Sign in with Shop lets a returning customer log in using their Shop app account, skipping manual account creation and speeding up checkout with saved details.

Steps

In your Shopify admin, go to Settings > Checkout and confirm Shop Pay and Sign in with Shop are enabled for your store — this is a Shopify-wide setting, not theme-specific.
In your Shopify admin, go to Online Store > Themes and click Customize on Force.
Select the Header section and confirm the account/sign-in icon is set to use this option, if Force offers a display setting for it.
Click Save.

This depends on Shop Pay being available in your store's country and currency — if the option isn't showing up, check your payment and checkout settings before assuming it's a theme issue.