The spell Crusader's Mantle is not part of the free Basic Rules / SRD. It is part of the Player's Handbook. If your character is in a campaign where somebody has purchased this spell or the Player's Handbook and somebody with a Master Tier subscription has enabled content sharing then you can use the spell for your character sheet. It does not, however, let you add the spell to homebrew until you have purchased the spell yourself.
If you have not purchased the spell by any means but in a campaign with somebody who has they can homebrew the item for you and you should then be able to select if for your character (homebrew items are shared between players of a campaign even without a master tier subscription).
If you're not in a campaign and have not purchased the spell then it is not possible: you will need to purchase the spell.
Cyb3rM1nd is correct. You have access to read the compendium content on the site and mobile application overall, and access to character sheet content in the campaign when content is shared with you. Sharing does not grant you access to content in the create homebrew section. You would need to own that content yourself to access it for homebrew creation.
Alternatively, as Cyb3rM1nd suggested, you could ask for the user who is sharing content with you to make the item for you, so you can access it on your character in the campaign.
Crusader's Mantle does not show up on the list for me to add to my sword. Why?
The spell Crusader's Mantle is not part of the free Basic Rules / SRD. It is part of the Player's Handbook. If your character is in a campaign where somebody has purchased this spell or the Player's Handbook and somebody with a Master Tier subscription has enabled content sharing then you can use the spell for your character sheet. It does not, however, let you add the spell to homebrew until you have purchased the spell yourself.
If you have not purchased the spell by any means but in a campaign with somebody who has they can homebrew the item for you and you should then be able to select if for your character (homebrew items are shared between players of a campaign even without a master tier subscription).
If you're not in a campaign and have not purchased the spell then it is not possible: you will need to purchase the spell.
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even if the phb is shared with me?
Cyb3rM1nd is correct. You have access to read the compendium content on the site and mobile application overall, and access to character sheet content in the campaign when content is shared with you. Sharing does not grant you access to content in the create homebrew section. You would need to own that content yourself to access it for homebrew creation.
Alternatively, as Cyb3rM1nd suggested, you could ask for the user who is sharing content with you to make the item for you, so you can access it on your character in the campaign.
thank both of you for the information, the alternative option works well and is one i did not consider.